For rapid diagnosis of Legionella in hospitalized patients, which test is commonly used?

• A. Urinary antigen test for Legionella pneumophila serogroup 1
• B. Blood culture on standard media
• C. Sputum Gram stain showing many organisms
• D. Chest CT with contrast

Answer: (A)

Rapid diagnosis of Legionella in hospitalized patients relies on the urinary antigen test for Legionella pneumophila serogroup 1. This test detects a bacterial antigen shed into the urine and can produce results within hours, making it the quickest and most practical option for initial diagnosis and prompt treatment decisions in a hospital setting. Its main advantage is high specificity, so a positive result is strongly indicative of LP1 infection, which is the most common cause of Legionnaires disease. The test is also easy to obtain—urine samples are noninvasive and routinely collected in hospitalized patients—and it’s widely available in clinical laboratories.

However, its sensitivity isn’t perfect, and it only detects serogroup 1 of Legionella pneumophila, so infections caused by other Legionella species or serogroups may be missed. When broader detection or confirmation is needed, or if LP1 is not suspected, other methods such as culture on specialized media or molecular testing (PCR) from respiratory samples can be used, though these are slower or more resource-intensive. Other options listed aren’t diagnostic for Legionella in the acute setting: blood culture on standard media usually won’t grow Legionella, sputum Gram stain lacks specificity for Legionella, and chest CT with contrast is an imaging study rather than a diagnostic test for Legionella infection.