A 58-year-old man who recently stayed in a hotel presents with high fever, cough, watery diarrhoea, and confusion. He is notably bradycardic for his fever. Sodium is 124 mmol/L, ALT is 90 IU/L, and chest radiograph shows patchy lobar consolidation unresponsive to amoxicillin. Which diagnostic test will most rapidly confirm the cause?

• A Paired acute and convalescent serology
• B Urinary antigen detection ✓
• C Sputum Gram stain showing gram-negative coccobacilli
• D Blood culture on chocolate agar

Explanation

Hyponatraemia, gastrointestinal symptoms, relative bradycardia, deranged liver enzymes, and failure of beta-lactam therapy in lobar pneumonia point to Legionella pneumophila. The Legionella urinary antigen test detects serogroup 1 within hours of presentation and is the fastest confirmatory test, guiding a switch to azithromycin or levofloxacin. Serology requires paired samples taken weeks apart, so it is retrospective. Legionella stains poorly with routine Gram stain and grows slowly on buffered charcoal yeast extract agar, not ordinary chocolate agar.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.