Community-acquired pneumonia

What you are treating

CAP is pneumonia acquired in the community — including nursing-home, dialysis, and clinic patients. HCAP is dead (ATS/, ; update AJRCCM 2025). Diagnosis is a new infiltrate plus a compatible syndrome (fever, cough, leukocytosis, sputum change, hypoxemia). Elderly blunted signs; neutropenic patients may have a clean film.

Triage, then severity

CURB-65
Confusion, BUN >19, RR >29, SBP <90 or DBP <60, age >65
0–1 can often go home. Fast, coarse, and what the ED actually remembers.
PSI
More accurate
Outpatient if <70, inpatient if >90. Use it when CURB-65 and the eyeball disagree.
Severe
ATS/IDSA
1 major (pressors or mechanical ventilation) or 3 minor: RR >30, P/F <250, multilobar, confusion, BUN >20, WBC <4 (not chemo), platelets <100, T <36°C, hypotension needing aggressive fluids.

Micro: S. pneumoniae and respiratory viruses first. Severe CAP adds S. aureus, Pseudomonas, Legionella (; ).
Lung ultrasound is a diagnostic alternative when the operator is competent.
Radiographic consolidation does not separate bacterial from viral.

Workup and regimens

Sputum Gram stain / culture (ET aspirate if intubated) if inpatient and severe or resistance risk. Adequate: >25 PMNs and <10 squamous cells. “Abundant squamous cells” is saliva.
Viral swab: influenza, COVID, RSV. Empiric antibiotics after a positive viral test in nonsevere CAP is individualized (AJRCCM 2025; CID 2025;ciaf625).
Blood cultures: positive in <10–20%. Get them (and sputum) if severe CAP, empiric MRSA/Pseudomonas, prior MRSA/Pseudomonas, or hospitalization with IV antibiotics ≤90 days.
Legionella urine antigen: Sn ~70%, Sp ~99%; serogroup 1 only (80–90% of US disease). Check if severe or recent exposure/travel. Clues: hyponatremia, high fever, diarrhea, travel ().
MRSA nasal PCR: NPV ~98%. Negative → stop MRSA coverage ().

*Heart, lung, liver, or kidney disease; diabetes; alcohol use disorder; malignancy; asplenia. Immunocompromised hosts need extra thought (Chest 2022;158:1896).

Setting	Preferred	Notes
Outpatient, no comorbidities, no MRSA/PsA risk	Amoxicillin 1 g TID, or doxycycline 100 mg BID, or a macrolide if local resistance <25%	US macrolide- and doxy-resistant pneumococcus is high. Do not default to a respiratory FQ.
Outpatient with comorbidities*	Amox-clav BID + azithromycin, or levofloxacin monotherapy	Cefpodoxime or cefuroxime can replace amox-clav; doxycycline can replace azithro. Levo is preferred if structural lung disease.
Inpatient, nonsevere	β-lactam (CTX) + macrolide (azithro), or levofloxacin monotherapy	Amp-sulbactam can replace CTX. Doxy can replace azithro if macrolide/FQ not tolerated.
Severe / ICU	β-lactam (CTX 1–2 g daily) + azithromycin or levofloxacin	In the ICU, azithromycin beats levo (anti-inflammatory signal).
MRSA / Pseudomonas risk	Vancomycin + cefepime (+ atypical coverage as above)	Cultures and a nasal swab to de-escalate. Never daptomycin for lung.

Resistance risk	Clues	Cover with
General	Hospitalization + IV antibiotics in 90 days; prior respiratory isolation of MRSA, Pseudomonas, or other MDRO	Add, then de-escalate
Pseudomonas	GNR on Gram stain, prior Pseudomonas, bronchiectasis, COPD with frequent antibiotic/steroid exacerbations	Cefepime, ceftazidime, pip-tazo, meropenem/imipenem. Double coverage is usually unnecessary in CAP.
MRSA	GPC clusters, recent influenza, necrotizing / cavitary disease / empyema, positive nares, ESRD, PWID, prior FQ	Vancomycin or linezolid
Anaerobes	Empyema or lung abscess only	Not garden-variety aspiration

Steroids, duration, pleural space

CAPE COD / ATS 2025

Hydrocortisone 200 mg/day cut day-28 death in ICU severe CAP (6.2% vs 11.9%; ). Population: P/F <300, PSI >130.
ATS 2025 now recommends steroids for severe CAP (AJRCCM 2025).
A meta-analysis: fewer intubations, no clear mortality win outside that trial ().

Do not

Steroids in influenza — associated with more death (Cochrane 2016).
Treat the film. CXR is still dirty at 1 month in half of patients (up to 12 weeks if older or COPD).
Observe 24 hours after the IV-to-PO switch. Convert when improving.

Expected cadence: tachycardia 2–3 days, fever 2–4, hypoxemia 3–6. No response at 72 h → CT ± BAL for empyema, abscess, fungus.
Parapneumonic effusion in 20–57% of pneumonia admissions (). Tap anything not tiny if infection is still on the table.

Stage	Fluid	Move
Uncomplicated	pH >7.2, glucose >60, Gram stain/culture negative	Antibiotics alone if <½ hemithorax and free-flowing. Therapeutic tap if symptomatic.
Complicated	pH <7.2, low glucose, still culture-negative, loculated, or large	Drain.
Empyema	Pus or positive Gram stain/culture	Chest tube. Also drain if loculation, >½ hemithorax, thickened parietal pleura, or sepsis with a pleural source ().