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

<i>Legionella</i> 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 <i>Legionella</i>, the most commonly encountered is <i>Legionella pneumophila</i>, 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 <i>Legionella</i> species, as cell-mediated immunity is vital in restricting intracellular growth. For detection, the least invasive and most commonly used test for <i>Legionella</i> is a urine antigen test (UAT). However, one major limitation of the UAT is that it only detects <i>Legionella pneumophila</i> 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 <i>Legionella</i> than just <i>Legionella</i> ​​​​​​<i>pneumophila</i> 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.