Professional and Linguistic Assessments Board (PLAB) Practice Exam

Which diagnostic test is used to confirm an infection with Legionella?

1. Blood culture

2. Sputum culture

3. Urinary antigen test

4. Chest X-ray

The correct answer is: Urinary antigen test

The urinary antigen test is the diagnostic test commonly used to confirm an infection with Legionella. This test detects the presence of specific antigens produced by Legionella bacteria in the urine, which is particularly useful because it allows for rapid diagnosis and does not require the collection of respiratory samples that are often difficult to obtain in patients with pneumonia. Legionella pneumonia can manifest similarly to other forms of pneumonia, which makes prompt and accurate diagnosis essential for effective treatment. The urinary antigen test offers high sensitivity and specificity for the most prevalent species of Legionella, namely Legionella pneumophila serogroup 1. In contrast, while blood cultures and sputum cultures can be used to diagnose bacterial infections, they are not particularly reliable for detecting Legionella. Blood cultures often yield negative results in Legionella pneumonia cases, and sputum cultures have lower sensitivity due to the difficulty in isolating the bacteria. A chest X-ray is primarily used for imaging and assessing lung pathology but does not confirm the infection itself. Therefore, the urinary antigen test stands out as the most effective method for confirming an infection with Legionella.