Synopsis
Identify the likely infectious cause of community-acquired pneumonia, obtain useful specimens, select proportionate empirical antibiotics, interpret microbiology safely and narrow, switch or stop treatment at a defined review point.
- Begin with host, exposure and resistance risk: recent antibiotics, previous resistant isolates, influenza, aspiration, immune suppression, travel, water-system exposure and clusters can change the pathogen hypothesis.
- Do not routinely request microbiology for uncomplicated low-severity adult CAP; in moderate or high severity, choose tests only when the result can refine treatment, detect a complication or trigger public-health action.
- When bacteraemia or sepsis is plausible, obtain at least two adult blood-culture sets using two aerobic and two anaerobic bottles, filling each with 8–10 mL and sending them promptly for incubation.
- A good lower-respiratory specimen is more useful than salivary sputum. Record previous antibiotics, request the relevant tests and assign a clinician to act on preliminary and post-discharge results.
- NICE first-line antibiotic choice follows severity, oral feasibility and allergy phenotype, then recent exposure, previous microbiology and the current local antibiogram modify the empirical plan.
- Review treatment and microbiology by 24–48 hours: confirm the diagnosis, narrow spectrum, stop unnecessary agents and switch intravenous therapy to oral when clinical response and absorption allow.
- For adults, plan to stop at five days unless a result requires longer treatment or clinical instability persists; write the indication, route, review time and stop rule when prescribing.
- Failure to improve is a new diagnostic problem. Look for empyema, abscess, obstruction, resistant or unexpected organisms, ineffective exposure and a non-infectious mimic before simply broadening antibiotics.
Key red flags
New fever, shock, haemoptysis, cavitation or rapid decline after apparent influenza improvement raises concern for bacterial coinfection, including Staphylococcus aureus, and needs urgent reassessment.
Travel accommodation, healthcare or workplace water exposure, hyponatraemia, diarrhoea, confusion or linked cases should prompt Legionella testing and early health-protection discussion rather than bedside diagnosis alone.
Neutropenia, transplantation, advanced HIV or substantial immunosuppression may blunt fever and expand the differential beyond routine CAP organisms, requiring early specialist sampling and treatment design.
Persistent fever, bacteraemia or deterioration despite therapy suggests inadequate exposure, resistance, an uncovered pathogen, infected pleural fluid, abscess or another source requiring investigation and control.
Investigation priorities
Avoid low-yield testing while identifying patients whose results could change treatment.
Management branches
CAP severity, sepsis physiology, host factors or an epidemiological clue makes microbiology or public-health action useful.
- Before antimicrobials when safe, collect only specimens that answer a defined question; record prior exposure, anatomical source, collection time and the clinician who owns each result.
- Institute pathogen-appropriate respiratory precautions immediately when influenza, COVID-19, measles, tuberculosis or another transmissible infection is plausible; do not wait for laboratory confirmation to protect others.
- For suspected Legionnaires’ disease, a linked cluster or another notifiable infection, contact the relevant UK health-protection route promptly and preserve travel, accommodation, workplace and water-exposure details.
A clinical diagnosis of CAP has been established and treatment must begin before definitive microbiology.
- Apply the current NICE adult severity table, using oral treatment first when feasible; clarify the allergy phenotype, pregnancy, renal and hepatic function, interactions and ability to absorb before selecting a regimen.