17:I[462708,["/_next/static/chunks/3yo3kiro3bmlp.js","/_next/static/chunks/3vr2wvd8f2efb.js","/_next/static/chunks/2vtmb68_vu-9t.js","/_next/static/chunks/2s8mt6_8wrpfo.js","/_next/static/chunks/3vkulz2i3l61y.js","/_next/static/chunks/1cqw2i1silz0i.js","/_next/static/chunks/14dbm3lm_t629.js","/_next/static/chunks/335_v2mb569ld.js","/_next/static/chunks/3ykn5ops9bft5.js","/_next/static/chunks/3cmavum774kjt.js","/_next/static/chunks/3th-0-a2xf9qs.js","/_next/static/chunks/0p0xpwde6kzmv.js","/_next/static/chunks/401-sxxnzbtf_.js","/_next/static/chunks/0wywtv698a1v0.js","/_next/static/chunks/2botzqwpnrzyj.js"],"ArticleReader"] 18:T5ccc,## Summary **Community-acquired pneumonia (CAP)** is an acute infection of the pulmonary parenchyma acquired outside the hospital setting. In adults, diagnosis generally requires a compatible acute respiratory syndrome plus a new pulmonary infiltrate on chest imaging; expert lung ultrasound is an acceptable diagnostic alternative to chest radiography where appropriate expertise is available.[1,2] Early management is organized around four decisions: **severity and oxygenation, site of care, microbiologic testing, and empiric antimicrobial selection**. The Pneumonia Severity Index (PSI) supports hospitalization decisions, but oxygen requirement, hemodynamics, oral intake, frailty, comorbidity, and social reliability can override a low calculated risk.[2] Severe CAP is defined by **one major ATS/IDSA criterion or at least three minor criteria**.[2] The 2025 ATS focused guideline, published in final journal form in 2026, changed several important practices.[1] Clinically stable nonsevere CAP can often receive **less than 5 days of antibiotics, with a minimum total course of 3 days**; severe CAP should receive **at least 5 days**.[1] A positive respiratory viral test does not automatically settle the antibacterial decision: selected otherwise healthy outpatients with convincing viral CAP and low probability of bacterial coinfection may be managed without empiric antibiotics, whereas hospitalized patients and many patients with significant comorbidity should initially receive antibacterial therapy.[1] Systemic corticosteroids are **not recommended for nonsevere CAP**. Current ATS and critical-care guidance supports corticosteroids in selected hospitalized adults with **severe bacterial CAP**, while influenza pneumonia remains an important exclusion from routine severe-CAP steroid use.[1,3] Trial regimens have varied; hydrocortisone 200 mg/day IV was used in a major severe-CAP randomized trial, so implementation should follow a local evidence-based protocol rather than improvised dosing.[3,4] Empiric antibiotics should be as narrow as safely possible. Do not use the retired “healthcare-associated pneumonia” label to trigger broad-spectrum therapy. Add MRSA or *Pseudomonas aeruginosa* coverage only when validated risk factors are present, especially **prior respiratory isolation** and relevant **recent hospitalization with parenteral antibiotic exposure**, interpreted with local epidemiology.[2] > **Caution:** Apparent early stability can deteriorate rapidly. Escalating oxygen requirement, respiratory rate ≥30/min, hypotension, altered mentation, rising lactate, multilobar disease, or inability to maintain oral intake should trigger immediate reassessment. ## Scope The standard CAP pathway is designed mainly for immunocompetent adults. Profound neutropenia, recent hematopoietic stem-cell or solid-organ transplantation, advanced cellular immunodeficiency, or major drug-induced immunosuppression changes the pathogen spectrum and usually requires a host-specific infectious-disease strategy.[2] CAP can be bacterial, viral, or mixed. Common bacterial pathogens include *Streptococcus pneumoniae*, *Haemophilus influenzae*, *Moraxella catarrhalis*, *Mycoplasma pneumoniae*, *Chlamydia pneumoniae*, and *Legionella* species. Respiratory viruses including influenza, SARS-CoV-2, RSV, human metapneumovirus, parainfluenza, adenovirus, and rhinovirus may cause pneumonia alone or coexist with bacterial infection.[1,2] ## Pathophysiology Microorganisms reach distal airways through microaspiration, inhalation, hematogenous spread, or rarely direct extension. Alveolar inflammation causes capillary leak and filling of airspaces with inflammatory cells and protein-rich fluid, producing ventilation-perfusion mismatch and, when extensive, physiologic shunt. Severe CAP can become a systemic inflammatory illness with vasodilation, endothelial dysfunction, myocardial stress, kidney injury, encephalopathy, septic shock, and acute respiratory distress syndrome. ## Clinical features ### Symptoms - cough, with or without sputum; - dyspnea; - fever or chills; - pleuritic chest pain; - fatigue and reduced appetite; - confusion, falls, or functional decline in older adults. ### Findings - tachypnea; - hypoxemia; - increased work of breathing; - focal crackles or bronchial breath sounds; - dullness to percussion or pleural findings; - confusion; - hypotension or impaired perfusion. No single symptom or auscultatory finding reliably confirms or excludes CAP. ## Red flags - new or rapidly increasing oxygen requirement; - severe work of breathing or exhaustion; - respiratory rate ≥30/min; - hypotension or impaired perfusion; - new confusion or reduced consciousness; - multilobar infiltrates; - hyperlactatemia with hypoperfusion; - inability to take oral fluids or medication; - major decompensation of heart, lung, kidney, or liver disease; - suspected empyema, abscess, necrotizing pneumonia, or post-obstructive disease. ## Diagnosis ### Imaging Chest radiography remains the usual initial study for hospitalized adults and for outpatients when diagnostic certainty or severity matters. CT is more sensitive and is useful when: - the chest radiograph is nondiagnostic despite strong suspicion; - complications are suspected; - a competing diagnosis such as pulmonary embolism or malignancy must be assessed; - focal opacity fails to resolve as expected. The current ATS guideline accepts **lung ultrasound** as an alternative to chest radiography when clinicians have adequate training.[1] Ultrasound can identify subpleural consolidation, dynamic air bronchograms, interstitial patterns, and pleural effusion, but centrally located disease may be missed. ### Laboratory assessment Testing should reflect severity rather than follow a fixed panel. Typical inpatient studies include: - CBC; - electrolytes and kidney function; - liver chemistry when relevant; - pulse oximetry; - lactate when shock or impaired perfusion is suspected; - blood gas when severe respiratory failure or uncertain ventilation/oxygenation requires it. **Procalcitonin should not be used alone to withhold initial antibiotics** when the clinical and imaging syndrome supports bacterial CAP because sensitivity is insufficient for that purpose.[2] ## Microbiologic testing Routine cultures are unnecessary in most mild outpatient CAP. Testing is more useful in severe disease, treatment failure, epidemiologic outbreaks, or when resistant pathogens are plausible. | Test | When useful | How it changes care | |---|---|---| | Blood cultures | Severe CAP; empiric MRSA/*Pseudomonas* therapy; selected complicated cases | Enables targeted therapy and identifies bacteremia | | Sputum Gram stain/culture | Severe CAP; prior resistant isolate; intubation; treatment failure | Supports narrowing or confirms resistant-pathogen therapy | | Respiratory viral NAAT | Seasonal circulation, hospitalization, outbreak, antiviral decision | Guides antiviral therapy, isolation, and bacterial-coinfection assessment | | *Legionella* urine antigen | Severe CAP, outbreak, relevant travel/epidemiology | Supports directed therapy; mainly detects serogroup 1 | | Pneumococcal urine antigen | Selected severe CAP | Can support narrowing in appropriate context | | MRSA nasal PCR | When anti-MRSA therapy is being considered | A negative result can support early de-escalation in many settings | Obtain cultures before broad-spectrum antibiotics when feasible, but do not delay urgent treatment in a critically ill patient. ## Differential diagnosis | Alternative | Clues favoring it | Immediate next step | |---|---|---| | Acute heart failure | Orthopnea, edema, elevated JVP, diffuse B-lines | Cardiac/lung ultrasound; heart-failure pathway | | Pulmonary embolism | Sudden dyspnea/pleuritic pain, VTE risk, disproportionate hypoxemia | PE probability-based pathway | | COPD/asthma exacerbation | Diffuse wheeze, known airway disease, no new infiltrate | Airway-disease treatment; assess for coexisting CAP | | Aspiration pneumonitis | Witnessed macroaspiration, abrupt hypoxemia, rapid early improvement possible | Supportive care; reassess for secondary infection | | Tuberculosis | Subacute course, weight loss, epidemiologic risk, cavitation | Airborne precautions and TB diagnostics | | Organizing pneumonia | Subacute symptoms, migratory opacities, poor antibiotic response | CT and pulmonary evaluation | | Diffuse alveolar hemorrhage | Falling hemoglobin, autoimmune context, hemoptysis may be absent | Urgent pulmonary/rheumatologic evaluation | | Malignancy/post-obstructive infection | Recurrent same-lobe disease, persistent opacity, weight loss | CT/bronchoscopy pathway | ## Severity and site of care ### Pneumonia Severity Index The ATS/IDSA framework prefers the **PSI** over CURB-65 as an aid to hospitalization decisions because of broader validation.[2] A score never substitutes for clinical judgment. Low predicted mortality does not justify discharge when the patient has hypoxemia, unstable comorbidity, inability to take oral therapy, severe frailty, rapid progression, or unreliable follow-up. ### Severe CAP criteria Severe CAP is present with **one major criterion or at least three minor criteria**.[2] **Major criteria** - septic shock requiring vasopressors; - respiratory failure requiring mechanical ventilation. **Minor criteria** - respiratory rate ≥30/min; - PaO2/FiO2 ≤250; - multilobar infiltrates; - confusion/disorientation; - BUN ≥20 mg/dL; - leukopenia <4,000/microliter attributable to infection; - platelet count <100,000/microliter; - core temperature <36°C; - hypotension requiring aggressive fluid resuscitation. A major criterion indicates ICU-level illness. Multiple minor criteria or rapidly worsening physiology warrants a monitored setting and early critical-care involvement. ## Initial stabilization | Domain | Assessment | Immediate action | |---|---|---| | Airway | Mental status, secretion burden, aspiration | Position/suction; secure airway if protection is inadequate | | Breathing | SpO2, respiratory rate, work of breathing | Oxygen; high-flow nasal oxygen or ventilatory support when needed | | Circulation | BP, perfusion, urine output, lactate | IV access, sepsis-appropriate resuscitation, vasopressor for persistent shock | | Infection control | Viral epidemiology and isolation needs | Appropriate precautions and respiratory testing | | Antimicrobials | Severity and resistant-pathogen risk | Prompt empiric therapy after key cultures when indicated | Do not delay appropriate antibiotics for nonessential diagnostics in septic shock or severe CAP. ## Empiric antimicrobial therapy Regimens must reflect local susceptibility, allergy history, recent antibiotics, prior cultures, kidney/liver function, QT risk, pregnancy, and drug interactions. ### Outpatient, no major comorbidity or resistant-pathogen risk Common ATS/IDSA options are:[2] - **amoxicillin 1 g orally three times daily**; or - **doxycycline 100 mg orally twice daily**; or - a macrolide only where local pneumococcal macrolide resistance is <25%, for example **azithromycin 500 mg on day 1, then 250 mg daily**. Under the 2025 ATS update, an otherwise healthy outpatient with a positive respiratory viral test and low probability of bacterial coinfection can sometimes be managed without empiric antibacterial therapy if follow-up is reliable.[1] ### Outpatient with comorbidity For significant chronic heart, lung, liver, or kidney disease; diabetes; alcohol use disorder; malignancy; or asplenia, options include:[2] - **amoxicillin/clavulanate 875/125 mg orally twice daily** plus azithromycin or doxycycline; or - **cefpodoxime 200 mg twice daily** or **cefuroxime 500 mg twice daily** plus azithromycin or doxycycline; or - a respiratory fluoroquinolone such as **levofloxacin 750 mg daily** or **moxifloxacin 400 mg daily** when benefits outweigh class-specific risks. A positive viral test does not routinely eliminate antibacterial treatment in this higher-risk group.[1] ### Hospitalized nonsevere CAP Common regimens include:[2] - **ceftriaxone 1–2 g IV every 24 hours**, **ampicillin/sulbactam 1.5–3 g IV every 6 hours**, **cefotaxime 1–2 g IV every 8 hours**, or **ceftaroline 600 mg IV every 12 hours**, plus **azithromycin 500 mg daily**; or - **levofloxacin 750 mg daily** or **moxifloxacin 400 mg daily** as monotherapy when appropriate; or - a beta-lactam plus **doxycycline 100 mg twice daily** when macrolides and fluoroquinolones are unsuitable. Adjust doses for kidney function and other patient-specific factors. ### Severe CAP Use a beta-lactam plus either a macrolide or a respiratory fluoroquinolone.[2] Beta-lactam monotherapy is not the preferred severe-CAP regimen. ## MRSA and Pseudomonas coverage Do not broaden therapy solely because of healthcare exposure. The strongest routinely used predictors are: - prior respiratory isolation of MRSA or *P. aeruginosa*; - recent hospitalization with parenteral antibiotics, especially when local epidemiology supports risk.[2] ### MRSA Options include: - vancomycin, dosed and monitored using a current institutional AUC-based protocol; or - **linezolid 600 mg IV/PO every 12 hours**.[2] ### Antipseudomonal beta-lactams Examples include:[2] - **piperacillin/tazobactam 4.5 g IV every 6 hours**; - **cefepime 2 g IV every 8 hours**; - **ceftazidime 2 g IV every 8 hours**; - **meropenem 1 g IV every 8 hours**; - **imipenem 500 mg IV every 6 hours**; - **aztreonam 2 g IV every 8 hours**. Obtain microbiology when broad coverage is started and de-escalate promptly if resistant pathogens are not supported. ## Viral CAP ### Influenza Treat influenza pneumonia with an appropriate antiviral when indicated, including hospitalized patients even when symptom duration exceeds 48 hours. Hospitalized influenza pneumonia generally receives empiric antibacterial therapy initially because bacterial coinfection may be occult.[2] ### Other respiratory viruses Current ATS guidance bases antibacterial decisions on severity, comorbidity, radiographic pattern, clinical trajectory, and bacterial-coinfection probability rather than the viral result alone.[1] ## Corticosteroids > **Current practice:** Evidence now supports corticosteroids for selected **severe bacterial CAP**, while routine use remains inappropriate in nonsevere CAP.[1,3] The 2024 Society of Critical Care Medicine update strongly recommended corticosteroids for hospitalized adults with severe bacterial CAP.[3] The current ATS guideline supports corticosteroids in severe CAP but not nonsevere CAP and excludes influenza pneumonia from the severe-CAP steroid recommendation.[1] A major randomized trial used **hydrocortisone 200 mg/day IV**, started early in severe CAP, with protocolized continuation/taper.[4] Because trials used different schedules, use a local evidence-based severe-CAP protocol rather than an arbitrary regimen. Consider contraindications and competing risks, including uncontrolled hyperglycemia, active GI bleeding, and suspected invasive fungal infection. ## Pleural infection and other complications Evaluate a parapneumonic effusion when it is moderate/large, loculated, associated with persistent fever or sepsis, or suspicious for empyema. Thoracentesis and pleural fluid testing guide drainage. Frank pus, positive pleural culture/Gram stain, or a complicated biochemical profile generally requires prompt drainage plus antibiotics. Other complications include: - lung abscess; - necrotizing pneumonia; - bacteremia and metastatic infection; - ARDS; - septic shock; - acute kidney injury; - myocardial injury and arrhythmia. Routine anaerobic coverage is not indicated merely because aspiration is suspected. Add anaerobic-directed therapy when lung abscess, empyema, or another clear anaerobic syndrome is present.[2] ## Reassessment and duration Assess daily for clinical stability: - improving heart and respiratory rates; - stable blood pressure; - improving oxygenation; - normalizing temperature; - improving mental status; - ability to eat and take oral medication.[1] For adults who reach clinical stability, the current ATS update suggests **<5 days with a minimum total duration of 3 days** for outpatient and hospitalized **nonsevere** CAP.[1] Severe CAP should receive **at least 5 days**.[1] Longer therapy is appropriate when there is: - empyema; - lung abscess; - necrotizing pneumonia; - bacteremia with a metastatic focus; - unusual pathogen requiring a longer course; - slow or incomplete clinical response. ### Failure to improve If the patient is not improving as expected, reassess rather than automatically extending or broadening antibiotics. Consider: - resistant or unusual pathogen; - inadequate source control; - empyema or abscess; - pulmonary embolism; - heart failure; - malignancy/post-obstructive disease; - organizing pneumonia; - inflammatory lung disease; - medication toxicity. Repeat imaging, CT, expanded microbiology, bronchoscopy, or specialist consultation may be appropriate. ## Disposition ### Outpatient care is reasonable when - oxygenation is adequate on baseline support; - hemodynamics are stable; - oral intake and medication are reliable; - severe comorbidity is not decompensated; - follow-up and return precautions are feasible. ### Admission is favored by - new oxygen requirement; - unstable vital signs; - inability to take oral treatment; - significant frailty or decompensated comorbidity; - failed outpatient therapy; - unreliable home support; - concerning complication. ### ICU/step-up care is indicated for - vasopressor-requiring shock; - invasive mechanical ventilation; - rapidly escalating oxygen support; - multiple severe-CAP minor criteria; - worsening acidosis, lactate, mental status, or organ dysfunction. ## Follow-up Routine follow-up chest imaging is not required in every adult who recovers clinically. Repeat imaging is appropriate when symptoms persist, the initial opacity was suspicious, resolution is unexpectedly slow, substantial lung-cancer risk is present, or an obstructing lesion is a concern.[2] Prevention includes: - smoking cessation; - influenza vaccination; - COVID-19 vaccination according to current public-health guidance; - age- and risk-appropriate pneumococcal vaccination. ## Special populations ### Pregnancy Do not withhold clinically necessary imaging. Beta-lactams and macrolides are commonly used; doxycycline and fluoroquinolones generally require an alternative unless specialist assessment supports their use. Severe disease warrants early obstetric and critical-care involvement. ### Older adults and frailty Confusion, falls, anorexia, and functional decline may dominate. PSI incorporates age-related risk but does not replace assessment of frailty, baseline function, aspiration risk, goals of care, and home support. ### Structural lung disease Bronchiectasis, cystic fibrosis, tracheostomy, and prior resistant isolates can alter the likely pathogen spectrum substantially. Use prior cultures and local microbiology rather than generic broadening. ## Acute management checklist - [ ] Confirm an acute respiratory syndrome plus a new infiltrate by chest radiography or expert lung ultrasound. - [ ] Measure oxygenation and identify shock or severe work of breathing. - [ ] Use PSI to support site-of-care decisions; separately assess severe-CAP criteria. - [ ] Obtain cultures before antibiotics when severe CAP or resistant-pathogen coverage is planned, without delaying urgent therapy. - [ ] Test for respiratory viruses when results affect antiviral, isolation, or antibacterial decisions. - [ ] Start the narrowest appropriate empiric regimen based on severity, comorbidity, prior cultures, and local resistance. - [ ] Add MRSA or antipseudomonal treatment only when validated risk is present. - [ ] Consider corticosteroids for severe bacterial CAP; avoid routine use in nonsevere CAP and influenza pneumonia. - [ ] Reassess daily for clinical stability, microbiology, IV-to-oral switch, and de-escalation. - [ ] Use short-course therapy only after stability is achieved: minimum 3 days for selected nonsevere CAP; at least 5 days for severe CAP. - [ ] Investigate complications or alternative diagnoses when expected improvement does not occur. ## Decision pathway ```text Acute respiratory syndrome + new infiltrate | +--> Assess oxygenation, shock, work of breathing +--> PSI + severe-CAP criteria | +--> Outpatient / nonsevere | +--> Narrow empiric therapy based on comorbidity/local resistance | +--> Viral-positive, otherwise healthy, low bacterial risk? | +--> Consider no antibacterial therapy with close follow-up | +--> Hospitalized / severe +--> Appropriate cultures + viral testing +--> Beta-lactam-based empiric therapy +--> Add MRSA/Pseudomonas coverage only if validated risk +--> Severe bacterial CAP -> consider corticosteroid protocol Daily reassessment | +--> Clinical stability -> narrow, switch route, determine short course +--> No improvement -> reassess diagnosis, resistance, source control, complications ``` ## Current practice changes The latest ATS guideline updates four clinically important areas:[1] 1. Expert lung ultrasound is acceptable as an alternative to chest radiography for diagnosis. 2. A positive viral test can justify withholding empiric antibiotics in selected otherwise healthy outpatients with low bacterial-coinfection probability, but not routinely in hospitalized patients. 3. Clinically stable nonsevere CAP can often be treated for fewer than 5 days, with a minimum of 3 days; severe CAP should receive at least 5 days. 4. Systemic corticosteroids are supported for selected severe CAP but not nonsevere CAP; influenza pneumonia is excluded from the severe-CAP steroid pathway. ## Related library topics - [Sepsis and Septic Shock](../critical-care/sepsis-and-septic-shock.md) - [Acute Respiratory Distress Syndrome](../critical-care/acute-respiratory-distress-syndrome.md) - [Acute Pulmonary Embolism](../pulmonology/acute-pulmonary-embolism.md) - [COPD Exacerbation](../pulmonology/copd-exacerbation.md) - [Febrile Neutropenia](../hematology-oncology/febrile-neutropenia.md) ## References 1. Jones BE, Ramirez JA, Oren E, et al. Diagnosis and Management of Community-acquired Pneumonia: An Official American Thoracic Society Clinical Practice Guideline. *Am J Respir Crit Care Med.* 2026;212(1):24-44. Published online July 18, 2025. doi:10.1164/rccm.202507-1692ST. PMID:40679934. https://pubmed.ncbi.nlm.nih.gov/40679934/ 2. Metlay JP, Waterer GW, Long AC, et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia: An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. *Am J Respir Crit Care Med.* 2019;200(7):e45-e67. doi:10.1164/rccm.201908-1581ST. PMID:31573350. https://www.idsociety.org/practice-guideline/community-acquired-pneumonia-cap-in-adults/ 3. Chaudhuri D, Nei AM, Rochwerg B, et al. 2024 Focused Update: Guidelines on Use of Corticosteroids in Sepsis, Acute Respiratory Distress Syndrome, and Community-Acquired Pneumonia. *Crit Care Med.* 2024;52(5):e219-e233. https://www.sccm.org/clinical-resources/guidelines/guidelines/use-of-corticosteroids-in-sepsis-ards-cap 4. Dequin PF, Meziani F, Quenot JP, et al. Hydrocortisone in Severe Community-Acquired Pneumonia. *N Engl J Med.* 2023;388:1931-1941. doi:10.1056/NEJMoa2215145. PMID:36942789.19:T5cec,## Summary **Community-acquired pneumonia (CAP)** is an acute infection of the pulmonary parenchyma acquired outside the hospital setting. In adults, diagnosis generally requires a compatible acute respiratory syndrome plus a new pulmonary infiltrate on chest imaging; expert lung ultrasound is an acceptable diagnostic alternative to chest radiography where appropriate expertise is available.[1,2] Early management is organized around four decisions: **severity and oxygenation, site of care, microbiologic testing, and empiric antimicrobial selection**. The Pneumonia Severity Index (PSI) supports hospitalization decisions, but oxygen requirement, hemodynamics, oral intake, frailty, comorbidity, and social reliability can override a low calculated risk.[2] Severe CAP is defined by **one major ATS/IDSA criterion or at least three minor criteria**.[2] The 2025 ATS focused guideline, published in final journal form in 2026, changed several important practices.[1] Clinically stable nonsevere CAP can often receive **less than 5 days of antibiotics, with a minimum total course of 3 days**; severe CAP should receive **at least 5 days**.[1] A positive respiratory viral test does not automatically settle the antibacterial decision: selected otherwise healthy outpatients with convincing viral CAP and low probability of bacterial coinfection may be managed without empiric antibiotics, whereas hospitalized patients and many patients with significant comorbidity should initially receive antibacterial therapy.[1] Systemic corticosteroids are **not recommended for nonsevere CAP**. Current ATS and critical-care guidance supports corticosteroids in selected hospitalized adults with **severe bacterial CAP**, while influenza pneumonia remains an important exclusion from routine severe-CAP steroid use.[1,3] Trial regimens have varied; hydrocortisone 200 mg/day IV was used in a major severe-CAP randomized trial, so implementation should follow a local evidence-based protocol rather than improvised dosing.[3,4] Empiric antibiotics should be as narrow as safely possible. Do not use the retired “healthcare-associated pneumonia” label to trigger broad-spectrum therapy. Add MRSA or *Pseudomonas aeruginosa* coverage only when validated risk factors are present, especially **prior respiratory isolation** and relevant **recent hospitalization with parenteral antibiotic exposure**, interpreted with local epidemiology.[2] > **Caution:** Apparent early stability can deteriorate rapidly. Escalating oxygen requirement, respiratory rate ≥30/min, hypotension, altered mentation, rising lactate, multilobar disease, or inability to maintain oral intake should trigger immediate reassessment. ## Scope The standard CAP pathway is designed mainly for immunocompetent adults. Profound neutropenia, recent hematopoietic stem-cell or solid-organ transplantation, advanced cellular immunodeficiency, or major drug-induced immunosuppression changes the pathogen spectrum and usually requires a host-specific infectious-disease strategy.[2] CAP can be bacterial, viral, or mixed. Common bacterial pathogens include *Streptococcus pneumoniae*, *Haemophilus influenzae*, *Moraxella catarrhalis*, *Mycoplasma pneumoniae*, *Chlamydia pneumoniae*, and *Legionella* species. Respiratory viruses including influenza, SARS-CoV-2, RSV, human metapneumovirus, parainfluenza, adenovirus, and rhinovirus may cause pneumonia alone or coexist with bacterial infection.[1,2] ## Pathophysiology Microorganisms reach distal airways through microaspiration, inhalation, hematogenous spread, or rarely direct extension. Alveolar inflammation causes capillary leak and filling of airspaces with inflammatory cells and protein-rich fluid, producing ventilation-perfusion mismatch and, when extensive, physiologic shunt. Severe CAP can become a systemic inflammatory illness with vasodilation, endothelial dysfunction, myocardial stress, kidney injury, encephalopathy, septic shock, and acute respiratory distress syndrome. ## Clinical features ### Symptoms - cough, with or without sputum; - dyspnea; - fever or chills; - pleuritic chest pain; - fatigue and reduced appetite; - confusion, falls, or functional decline in older adults. ### Findings - tachypnea; - hypoxemia; - increased work of breathing; - focal crackles or bronchial breath sounds; - dullness to percussion or pleural findings; - confusion; - hypotension or impaired perfusion. No single symptom or auscultatory finding reliably confirms or excludes CAP. ## Red flags - new or rapidly increasing oxygen requirement; - severe work of breathing or exhaustion; - respiratory rate ≥30/min; - hypotension or impaired perfusion; - new confusion or reduced consciousness; - multilobar infiltrates; - hyperlactatemia with hypoperfusion; - inability to take oral fluids or medication; - major decompensation of heart, lung, kidney, or liver disease; - suspected empyema, abscess, necrotizing pneumonia, or post-obstructive disease. ## Diagnosis ### Imaging Chest radiography remains the usual initial study for hospitalized adults and for outpatients when diagnostic certainty or severity matters. CT is more sensitive and is useful when: - the chest radiograph is nondiagnostic despite strong suspicion; - complications are suspected; - a competing diagnosis such as pulmonary embolism or malignancy must be assessed; - focal opacity fails to resolve as expected. The current ATS guideline accepts **lung ultrasound** as an alternative to chest radiography when clinicians have adequate training.[1] Ultrasound can identify subpleural consolidation, dynamic air bronchograms, interstitial patterns, and pleural effusion, but centrally located disease may be missed. ### Laboratory assessment Testing should reflect severity rather than follow a fixed panel. Typical inpatient studies include: - CBC; - electrolytes and kidney function; - liver chemistry when relevant; - pulse oximetry; - lactate when shock or impaired perfusion is suspected; - blood gas when severe respiratory failure or uncertain ventilation/oxygenation requires it. **Procalcitonin should not be used alone to withhold initial antibiotics** when the clinical and imaging syndrome supports bacterial CAP because sensitivity is insufficient for that purpose.[2] ## Microbiologic testing Routine cultures are unnecessary in most mild outpatient CAP. Testing is more useful in severe disease, treatment failure, epidemiologic outbreaks, or when resistant pathogens are plausible. | Test | When useful | How it changes care | |---|---|---| | Blood cultures | Severe CAP; empiric MRSA/*Pseudomonas* therapy; selected complicated cases | Enables targeted therapy and identifies bacteremia | | Sputum Gram stain/culture | Severe CAP; prior resistant isolate; intubation; treatment failure | Supports narrowing or confirms resistant-pathogen therapy | | Respiratory viral NAAT | Seasonal circulation, hospitalization, outbreak, antiviral decision | Guides antiviral therapy, isolation, and bacterial-coinfection assessment | | *Legionella* urine antigen | Severe CAP, outbreak, relevant travel/epidemiology | Supports directed therapy; mainly detects serogroup 1 | | Pneumococcal urine antigen | Selected severe CAP | Can support narrowing in appropriate context | | MRSA nasal PCR | When anti-MRSA therapy is being considered | A negative result can support early de-escalation in many settings | Obtain cultures before broad-spectrum antibiotics when feasible, but do not delay urgent treatment in a critically ill patient. ## Differential diagnosis | Alternative | Clues favoring it | Immediate next step | |---|---|---| | Acute heart failure | Orthopnea, edema, elevated JVP, diffuse B-lines | Cardiac/lung ultrasound; heart-failure pathway | | Pulmonary embolism | Sudden dyspnea/pleuritic pain, VTE risk, disproportionate hypoxemia | PE probability-based pathway | | COPD/asthma exacerbation | Diffuse wheeze, known airway disease, no new infiltrate | Airway-disease treatment; assess for coexisting CAP | | Aspiration pneumonitis | Witnessed macroaspiration, abrupt hypoxemia, rapid early improvement possible | Supportive care; reassess for secondary infection | | Tuberculosis | Subacute course, weight loss, epidemiologic risk, cavitation | Airborne precautions and TB diagnostics | | Organizing pneumonia | Subacute symptoms, migratory opacities, poor antibiotic response | CT and pulmonary evaluation | | Diffuse alveolar hemorrhage | Falling hemoglobin, autoimmune context, hemoptysis may be absent | Urgent pulmonary/rheumatologic evaluation | | Malignancy/post-obstructive infection | Recurrent same-lobe disease, persistent opacity, weight loss | CT/bronchoscopy pathway | ## Severity and site of care ### Pneumonia Severity Index The ATS/IDSA framework prefers the **PSI** over CURB-65 as an aid to hospitalization decisions because of broader validation.[2] A score never substitutes for clinical judgment. Low predicted mortality does not justify discharge when the patient has hypoxemia, unstable comorbidity, inability to take oral therapy, severe frailty, rapid progression, or unreliable follow-up. ### Severe CAP criteria Severe CAP is present with **one major criterion or at least three minor criteria**.[2] **Major criteria** - septic shock requiring vasopressors; - respiratory failure requiring mechanical ventilation. **Minor criteria** - respiratory rate ≥30/min; - PaO2/FiO2 ≤250; - multilobar infiltrates; - confusion/disorientation; - BUN ≥20 mg/dL; - leukopenia <4,000/microliter attributable to infection; - platelet count <100,000/microliter; - core temperature <36°C; - hypotension requiring aggressive fluid resuscitation. A major criterion indicates ICU-level illness. Multiple minor criteria or rapidly worsening physiology warrants a monitored setting and early critical-care involvement. ## Initial stabilization | Domain | Assessment | Immediate action | |---|---|---| | Airway | Mental status, secretion burden, aspiration | Position/suction; secure airway if protection is inadequate | | Breathing | SpO2, respiratory rate, work of breathing | Oxygen; high-flow nasal oxygen or ventilatory support when needed | | Circulation | BP, perfusion, urine output, lactate | IV access, sepsis-appropriate resuscitation, vasopressor for persistent shock | | Infection control | Viral epidemiology and isolation needs | Appropriate precautions and respiratory testing | | Antimicrobials | Severity and resistant-pathogen risk | Prompt empiric therapy after key cultures when indicated | Do not delay appropriate antibiotics for nonessential diagnostics in septic shock or severe CAP. ## Empiric antimicrobial therapy Regimens must reflect local susceptibility, allergy history, recent antibiotics, prior cultures, kidney/liver function, QT risk, pregnancy, and drug interactions. ### Outpatient, no major comorbidity or resistant-pathogen risk Common ATS/IDSA options are:[2] - **amoxicillin 1 g orally three times daily**; or - **doxycycline 100 mg orally twice daily**; or - a macrolide only where local pneumococcal macrolide resistance is <25%, for example **azithromycin 500 mg on day 1, then 250 mg daily**. Under the 2025 ATS update, an otherwise healthy outpatient with a positive respiratory viral test and low probability of bacterial coinfection can sometimes be managed without empiric antibacterial therapy if follow-up is reliable.[1] ### Outpatient with comorbidity For significant chronic heart, lung, liver, or kidney disease; diabetes; alcohol use disorder; malignancy; or asplenia, options include:[2] - **amoxicillin/clavulanate 875/125 mg orally twice daily** plus azithromycin or doxycycline; or - **cefpodoxime 200 mg twice daily** or **cefuroxime 500 mg twice daily** plus azithromycin or doxycycline; or - a respiratory fluoroquinolone such as **levofloxacin 750 mg daily** or **moxifloxacin 400 mg daily** when benefits outweigh class-specific risks. A positive viral test does not routinely eliminate antibacterial treatment in this higher-risk group.[1] ### Hospitalized nonsevere CAP Common regimens include:[2] - **ceftriaxone 1–2 g IV every 24 hours**, **ampicillin/sulbactam 1.5–3 g IV every 6 hours**, **cefotaxime 1–2 g IV every 8 hours**, or **ceftaroline 600 mg IV every 12 hours**, plus **azithromycin 500 mg daily**; or - **levofloxacin 750 mg daily** or **moxifloxacin 400 mg daily** as monotherapy when appropriate; or - a beta-lactam plus **doxycycline 100 mg twice daily** when macrolides and fluoroquinolones are unsuitable. Adjust doses for kidney function and other patient-specific factors. ### Severe CAP Use a beta-lactam plus either a macrolide or a respiratory fluoroquinolone.[2] Beta-lactam monotherapy is not the preferred severe-CAP regimen. ## MRSA and Pseudomonas coverage Do not broaden therapy solely because of healthcare exposure. The strongest routinely used predictors are: - prior respiratory isolation of MRSA or *P. aeruginosa*; - recent hospitalization with parenteral antibiotics, especially when local epidemiology supports risk.[2] ### MRSA Options include: - vancomycin, dosed and monitored using a current institutional AUC-based protocol; or - **linezolid 600 mg IV/PO every 12 hours**.[2] ### Antipseudomonal beta-lactams Examples include:[2] - **piperacillin/tazobactam 4.5 g IV every 6 hours**; - **cefepime 2 g IV every 8 hours**; - **ceftazidime 2 g IV every 8 hours**; - **meropenem 1 g IV every 8 hours**; - **imipenem 500 mg IV every 6 hours**; - **aztreonam 2 g IV every 8 hours**. Obtain microbiology when broad coverage is started and de-escalate promptly if resistant pathogens are not supported. ## Viral CAP ### Influenza Treat influenza pneumonia with an appropriate antiviral when indicated, including hospitalized patients even when symptom duration exceeds 48 hours. Hospitalized influenza pneumonia generally receives empiric antibacterial therapy initially because bacterial coinfection may be occult.[2] ### Other respiratory viruses Current ATS guidance bases antibacterial decisions on severity, comorbidity, radiographic pattern, clinical trajectory, and bacterial-coinfection probability rather than the viral result alone.[1] ## Corticosteroids > **Current practice:** Evidence now supports corticosteroids for selected **severe bacterial CAP**, while routine use remains inappropriate in nonsevere CAP.[1,3] The 2024 Society of Critical Care Medicine update strongly recommended corticosteroids for hospitalized adults with severe bacterial CAP.[3] The current ATS guideline supports corticosteroids in severe CAP but not nonsevere CAP and excludes influenza pneumonia from the severe-CAP steroid recommendation.[1] A major randomized trial used **hydrocortisone 200 mg/day IV**, started early in severe CAP, with protocolized continuation/taper.[4] Because trials used different schedules, use a local evidence-based severe-CAP protocol rather than an arbitrary regimen. Consider contraindications and competing risks, including uncontrolled hyperglycemia, active GI bleeding, and suspected invasive fungal infection. ## Pleural infection and other complications Evaluate a parapneumonic effusion when it is moderate/large, loculated, associated with persistent fever or sepsis, or suspicious for empyema. Thoracentesis and pleural fluid testing guide drainage. Frank pus, positive pleural culture/Gram stain, or a complicated biochemical profile generally requires prompt drainage plus antibiotics. Other complications include: - lung abscess; - necrotizing pneumonia; - bacteremia and metastatic infection; - ARDS; - septic shock; - [acute kidney injury](/library/acute-kidney-injury); - myocardial injury and arrhythmia. Routine anaerobic coverage is not indicated merely because aspiration is suspected. Add anaerobic-directed therapy when lung abscess, empyema, or another clear anaerobic syndrome is present.[2] ## Reassessment and duration Assess daily for clinical stability: - improving heart and respiratory rates; - stable blood pressure; - improving oxygenation; - normalizing temperature; - improving mental status; - ability to eat and take oral medication.[1] For adults who reach clinical stability, the current ATS update suggests **<5 days with a minimum total duration of 3 days** for outpatient and hospitalized **nonsevere** CAP.[1] Severe CAP should receive **at least 5 days**.[1] Longer therapy is appropriate when there is: - empyema; - lung abscess; - necrotizing pneumonia; - bacteremia with a metastatic focus; - unusual pathogen requiring a longer course; - slow or incomplete clinical response. ### Failure to improve If the patient is not improving as expected, reassess rather than automatically extending or broadening antibiotics. Consider: - resistant or unusual pathogen; - inadequate source control; - empyema or abscess; - pulmonary embolism; - heart failure; - malignancy/post-obstructive disease; - organizing pneumonia; - inflammatory lung disease; - medication toxicity. Repeat imaging, CT, expanded microbiology, bronchoscopy, or specialist consultation may be appropriate. ## Disposition ### Outpatient care is reasonable when - oxygenation is adequate on baseline support; - hemodynamics are stable; - oral intake and medication are reliable; - severe comorbidity is not decompensated; - follow-up and return precautions are feasible. ### Admission is favored by - new oxygen requirement; - unstable vital signs; - inability to take oral treatment; - significant frailty or decompensated comorbidity; - failed outpatient therapy; - unreliable home support; - concerning complication. ### ICU/step-up care is indicated for - vasopressor-requiring shock; - invasive mechanical ventilation; - rapidly escalating oxygen support; - multiple severe-CAP minor criteria; - worsening acidosis, lactate, mental status, or organ dysfunction. ## Follow-up Routine follow-up chest imaging is not required in every adult who recovers clinically. Repeat imaging is appropriate when symptoms persist, the initial opacity was suspicious, resolution is unexpectedly slow, substantial lung-cancer risk is present, or an obstructing lesion is a concern.[2] Prevention includes: - smoking cessation; - influenza vaccination; - COVID-19 vaccination according to current public-health guidance; - age- and risk-appropriate pneumococcal vaccination. ## Special populations ### Pregnancy Do not withhold clinically necessary imaging. Beta-lactams and macrolides are commonly used; doxycycline and fluoroquinolones generally require an alternative unless specialist assessment supports their use. Severe disease warrants early obstetric and critical-care involvement. ### Older adults and frailty Confusion, falls, anorexia, and functional decline may dominate. PSI incorporates age-related risk but does not replace assessment of frailty, baseline function, aspiration risk, goals of care, and home support. ### Structural lung disease Bronchiectasis, cystic fibrosis, tracheostomy, and prior resistant isolates can alter the likely pathogen spectrum substantially. Use prior cultures and local microbiology rather than generic broadening. ## Acute management checklist - [ ] Confirm an acute respiratory syndrome plus a new infiltrate by chest radiography or expert lung ultrasound. - [ ] Measure oxygenation and identify shock or severe work of breathing. - [ ] Use PSI to support site-of-care decisions; separately assess severe-CAP criteria. - [ ] Obtain cultures before antibiotics when severe CAP or resistant-pathogen coverage is planned, without delaying urgent therapy. - [ ] Test for respiratory viruses when results affect antiviral, isolation, or antibacterial decisions. - [ ] Start the narrowest appropriate empiric regimen based on severity, comorbidity, prior cultures, and local resistance. - [ ] Add MRSA or antipseudomonal treatment only when validated risk is present. - [ ] Consider corticosteroids for severe bacterial CAP; avoid routine use in nonsevere CAP and influenza pneumonia. - [ ] Reassess daily for clinical stability, microbiology, IV-to-oral switch, and de-escalation. - [ ] Use short-course therapy only after stability is achieved: minimum 3 days for selected nonsevere CAP; at least 5 days for severe CAP. - [ ] Investigate complications or alternative diagnoses when expected improvement does not occur. ## Decision pathway ```text Acute respiratory syndrome + new infiltrate | +--> Assess oxygenation, shock, work of breathing +--> PSI + severe-CAP criteria | +--> Outpatient / nonsevere | +--> Narrow empiric therapy based on comorbidity/local resistance | +--> Viral-positive, otherwise healthy, low bacterial risk? | +--> Consider no antibacterial therapy with close follow-up | +--> Hospitalized / severe +--> Appropriate cultures + viral testing +--> Beta-lactam-based empiric therapy +--> Add MRSA/Pseudomonas coverage only if validated risk +--> Severe bacterial CAP -> consider corticosteroid protocol Daily reassessment | +--> Clinical stability -> narrow, switch route, determine short course +--> No improvement -> reassess diagnosis, resistance, source control, complications ``` ## Current practice changes The latest ATS guideline updates four clinically important areas:[1] 1. Expert lung ultrasound is acceptable as an alternative to chest radiography for diagnosis. 2. A positive viral test can justify withholding empiric antibiotics in selected otherwise healthy outpatients with low bacterial-coinfection probability, but not routinely in hospitalized patients. 3. Clinically stable nonsevere CAP can often be treated for fewer than 5 days, with a minimum of 3 days; severe CAP should receive at least 5 days. 4. Systemic corticosteroids are supported for selected severe CAP but not nonsevere CAP; influenza pneumonia is excluded from the severe-CAP steroid pathway. ## Related library topics - [Sepsis and Septic Shock](../critical-care/sepsis-and-septic-shock.md) - [Acute Respiratory Distress Syndrome](../critical-care/acute-respiratory-distress-syndrome.md) - [Acute Pulmonary Embolism](../pulmonology/acute-pulmonary-embolism.md) - [COPD Exacerbation](../pulmonology/copd-exacerbation.md) - [Febrile Neutropenia](../hematology-oncology/febrile-neutropenia.md) ## References 1. Jones BE, Ramirez JA, Oren E, et al. Diagnosis and Management of Community-acquired Pneumonia: An Official American Thoracic Society Clinical Practice Guideline. *Am J Respir Crit Care Med.* 2026;212(1):24-44. Published online July 18, 2025. doi:10.1164/rccm.202507-1692ST. PMID:40679934. https://pubmed.ncbi.nlm.nih.gov/40679934/ 2. Metlay JP, Waterer GW, Long AC, et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia: An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. *Am J Respir Crit Care Med.* 2019;200(7):e45-e67. doi:10.1164/rccm.201908-1581ST. PMID:31573350. https://www.idsociety.org/practice-guideline/community-acquired-pneumonia-cap-in-adults/ 3. Chaudhuri D, Nei AM, Rochwerg B, et al. 2024 Focused Update: Guidelines on Use of Corticosteroids in Sepsis, Acute Respiratory Distress Syndrome, and Community-Acquired Pneumonia. *Crit Care Med.* 2024;52(5):e219-e233. https://www.sccm.org/clinical-resources/guidelines/guidelines/use-of-corticosteroids-in-sepsis-ards-cap 4. Dequin PF, Meziani F, Quenot JP, et al. Hydrocortisone in Severe Community-Acquired Pneumonia. *N Engl J Med.* 2023;388:1931-1941. doi:10.1056/NEJMoa2215145. 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In adults, diagnosis generally requires a compatible acute respiratory syndrome plus a new pu…","aliases":[],"category":"Infectious Disease","tags":["Infectious Disease","Pulmonology"],"updatedAt":"$undefined","body":"$18","headings":[{"id":"summary","text":"Summary","level":2},{"id":"scope","text":"Scope","level":2},{"id":"pathophysiology","text":"Pathophysiology","level":2},{"id":"clinical-features","text":"Clinical features","level":2},{"id":"symptoms","text":"Symptoms","level":3},{"id":"findings","text":"Findings","level":3},{"id":"red-flags","text":"Red flags","level":2},{"id":"diagnosis","text":"Diagnosis","level":2},{"id":"imaging","text":"Imaging","level":3},{"id":"laboratory-assessment","text":"Laboratory assessment","level":3},{"id":"microbiologic-testing","text":"Microbiologic testing","level":2},{"id":"differential-diagnosis","text":"Differential diagnosis","level":2},{"id":"severity-and-site-of-care","text":"Severity and site of care","level":2},{"id":"pneumonia-severity-index","text":"Pneumonia Severity Index","level":3},{"id":"severe-cap-criteria","text":"Severe CAP criteria","level":3},{"id":"initial-stabilization","text":"Initial stabilization","level":2},{"id":"empiric-antimicrobial-therapy","text":"Empiric antimicrobial therapy","level":2},{"id":"outpatient-no-major-comorbidity-or-resistant-pathogen-risk","text":"Outpatient, no major comorbidity or resistant-pathogen risk","level":3},{"id":"outpatient-with-comorbidity","text":"Outpatient with comorbidity","level":3},{"id":"hospitalized-nonsevere-cap","text":"Hospitalized nonsevere CAP","level":3},{"id":"severe-cap","text":"Severe CAP","level":3},{"id":"mrsa-and-pseudomonas-coverage","text":"MRSA and Pseudomonas coverage","level":2},{"id":"mrsa","text":"MRSA","level":3},{"id":"antipseudomonal-beta-lactams","text":"Antipseudomonal beta-lactams","level":3},{"id":"viral-cap","text":"Viral CAP","level":2},{"id":"influenza","text":"Influenza","level":3},{"id":"other-respiratory-viruses","text":"Other respiratory viruses","level":3},{"id":"corticosteroids","text":"Corticosteroids","level":2},{"id":"pleural-infection-and-other-complications","text":"Pleural infection and other complications","level":2},{"id":"reassessment-and-duration","text":"Reassessment and duration","level":2},{"id":"failure-to-improve","text":"Failure to improve","level":3},{"id":"disposition","text":"Disposition","level":2},{"id":"outpatient-care-is-reasonable-when","text":"Outpatient care is reasonable when","level":3},{"id":"admission-is-favored-by","text":"Admission is favored by","level":3},{"id":"icustep-up-care-is-indicated-for","text":"ICU/step-up care is indicated for","level":3},{"id":"follow-up","text":"Follow-up","level":2},{"id":"special-populations","text":"Special populations","level":2},{"id":"pregnancy","text":"Pregnancy","level":3},{"id":"older-adults-and-frailty","text":"Older adults and frailty","level":3},{"id":"structural-lung-disease","text":"Structural lung disease","level":3},{"id":"acute-management-checklist","text":"Acute management checklist","level":2},{"id":"decision-pathway","text":"Decision pathway","level":2},{"id":"current-practice-changes","text":"Current practice changes","level":2},{"id":"related-library-topics","text":"Related library topics","level":2},{"id":"references","text":"References","level":2}],"readingMinutes":14},"linkedMarkdown":"$19","backlinks":[],"related":[],"previewBySlug":{"abcde-approach":{"title":"ABCDE Approach","description":"The ABCDE approach is a structured method for the immediate assessment and treatment of a deteriorating or critically ill patient: Airway, Breathing, Circulation, Disability, Exposure. 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