Legionella pneumophila serogroup 3 infection: importance of serology.

We present a case of Legionella pneumophila serogroup 3 (LP3) infection in a patient with severe community-acquired pneumonia (CAP). The diagnosis was complicated by an initial equivocal L. pneumophila urinary antigen test, followed by two negative samples. LP3 was cultured from a sputum sample and the diagnosis was confirmed by serology 15 days into the admission. This case highlights the importance of considering non-LP1 serogroups as causes of CAP and the role of serological testing in diagnosis.