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Severe travel-associated community-acquired pneumonia with suspected Legionnaires' disease — ABIM Board MCQ

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HardPneumoniaSevere travel-associated community-acquired pneumonia with suspected Legionnaires' diseaseABIM Board

A 62-year-old man with COPD is admitted with 3 days of fever, nonproductive cough, diarrhea, and progressive dyspnea that began 6 days after returning from a hotel conference. Temperature is 39.4°C (102.9°F), blood pressure is 78/46 mm Hg, and oxygen saturation is 82% despite supplemental oxygen. Chest radiography shows bilateral multifocal airspace opacities. He is intubated, receives norepinephrine, and is started on ceftriaxone plus azithromycin. Serum sodium is 126 mEq/L. A Legionella urinary antigen test is negative. The health department reports that another conference attendee has laboratory-confirmed Legionnaires' disease. Which additional diagnostic test is most important for confirming the suspected etiology and supporting investigation of a common source?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: B — Send an endotracheal aspirate for Legionella culture on selective media

Explanation lettering: E = shown as A · D = shown as B · A = shown as C · B = shown as D · C = shown as E

This patient has severe community-acquired pneumonia, defined here by both mechanical ventilation and vasopressor-dependent shock, as well as a compelling epidemiologic link to a travel-associated Legionella outbreak. A negative urinary antigen test does not exclude Legionnaires' disease because this assay typically detects only Legionella pneumophila serogroup 1. Lower respiratory secretions should therefore be submitted specifically for Legionella culture on selective media; routine respiratory culture does not reliably recover Legionella. Culture can detect additional species and serogroups and yields a clinical isolate that can be compared molecularly with environmental isolates during source investigation. Repeating the urinary antigen test (A) does not address its limited organism coverage. Paired serology (B) is slow, may be nonspecific, and cannot guide acute management or provide an isolate for source comparison. Routine bacterial culture alone (C) is appropriate in intubated severe pneumonia but requires a Legionella-specific request and selective media to evaluate this suspected pathogen. Environmental sampling (E) is an important public health measure, but it should occur in parallel and must not delay appropriate clinical specimen collection. Antibiotic treatment likewise should not be delayed while specimens are obtained.

Reference: Clinical Guidance for Legionella Infections (June 9, 2025) — https://www.cdc.gov/legionella/hcp/clinical-guidance/index.html Methods for Legionella Testing and Specimen Collection (May 15, 2026) — https://www.cdc.gov/investigate-legionella/php/public-health-strategy/legionella-testing.html Diagnosis and Treatment of Adults with Community-acquired Pneumonia (2019) — https://www.idsociety.org/globalassets/idsa/practice-guidelines/community-acquired-pneumonia-in-adults/executive_summary.pdf