What is the risk of missing legionellosis relying on urinary antigen testing solely? A retrospective Belgian multicenter study.

Currently, diagnosis of legionellosis relies mainly on urinary antigen testing (UAT) for Legionella pneumophila serogroup 1 (Lp1). However, this test has several limitations, particularly missing non-Lp1 infections. The purpose of this large multicenter study was to investigate the risk of missing legionellosis relying on UAT solely. Molecular results of Legionella detection as part of a first-line (syndromic) testing algorithm for severe respiratory tract infections were investigated retrospectively and compared with UAT results in 14 Belgian laboratories. Overall, 44.4% (20/45) UAT results appeared false negative and were reclassified as legionellosis based on PCR findings [Legionnaires' disease, 37.5% (15/40); Pontiac fever, 100% (5/5)]. A total of 39.4% (26/66) diagnosis probably would have been missed or delayed without a syndromic approach, as UAT or specific molecular testing for Legionella was not requested by the clinician. Furthermore, we confirmed the higher sensitivity of molecular Legionella detection in lower respiratory tract compared with upper respiratory tract specimens (p = 0.010).