# A Case of Polymerase Chain Reaction (PCR)-Positive Urine Antigen and Culture-Negative Legionnaires’ Disease in an Immunosuppressed Patient

## TL;DR

This case report shows how PCR testing can detect Legionnaires' disease in an immunosuppressed patient when other tests fail.

## Contribution

Highlights the importance of PCR testing in diagnosing Legionnaires' disease in high-risk immunosuppressed patients.

## Key findings

- Urine antigen test failed to detect Legionella in the patient.
- PCR testing confirmed Legionella despite negative culture results.
- Immunosuppressed patients may require more sensitive diagnostic methods.

## Abstract

Legionella is an aerobic, intracellular, Gram-negative bacillus native to warm water sources and soils and is transmitted to humans via aerosols. While there are numerous different species of Legionella, the most commonly encountered is Legionella pneumophila, particularly serotype 1. This bacteriumnotably causes a mild, self-limited infection known as Pontiac fever, in addition to a more severe form of pneumonia calledLegionnaires' disease. Like any pneumonia, Legionnaires’ disease carries a mortality risk, making early and rapid detection crucial. This is of particular importance in immunosuppressed patients, as they possess a weakened cell-mediated defense. As with any intracellular organism, host immune defense plays a crucial role in the protection against the Legionella species, as cell-mediated immunity is vital in restricting intracellular growth. For detection, the least invasive and most commonly used test for Legionella is a urine antigen test (UAT). However, one major limitation of the UAT is that it only detects Legionella pneumophila serotype 1 antigen, which can lead to false negatives. Cultures, typically obtained from respiratory secretions, are usually the next diagnostic test used due to their ability to detect more species of Legionella than just Legionella ​​​​​​pneumophila serotype 1. However, while broader than UAT, cultures lack sensitivity and can take days to show any growth. Given the limitations of these first-line diagnostic tests, the following case report highlights that further testing with polymerase chain reaction (PCR) may be necessary to definitively rule out Legionnaires’ disease in already high-risk, immunosuppressed patients when clinical suspicion is strong.