Legionella Pneumonia — RACP Adult Medicine MCQ

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ModerateInfectious DiseasesLegionella PneumoniaRACP Adult Medicine

A 55-year-old man with community-acquired pneumonia has a positive urinary Legionella antigen. He is haemodynamically stable but requires supplemental oxygen. What is the recommended antibiotic treatment?

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Correct answer: B — Azithromycin 500 mg daily IV/oral or doxycycline 100 mg BD

Legionella pneumophila pneumonia requires treatment with macrolides (azithromycin preferred), fluoroquinolones (levofloxacin, moxifloxacin), or doxycycline – agents that achieve adequate intracellular concentrations (Legionella is a facultative intracellular organism). Beta-lactams (amoxicillin, ceftriaxone) are INEFFECTIVE against Legionella despite being standard CAP antibiotics. The urinary antigen test only detects serogroup 1 (~80% of cases). Treatment duration is typically 7–14 days (longer for severe disease or immunosuppressed patients).

Reference: eTG – 2025 – Antibiotic Guidelines