Polymerase chain reaction (PCR) testing may be superior to urinary antigen testing when diagnosing legionellosis, according to a study published in the European Journal of Clinical Microbiology & Infectious Diseases.

The Legionella genus includes more than 50 species and 70 serogroups of bacteria, with almost half of the species being associated with human disease. Legionella pneumophila serogroup 1 (Lp1) is the most common cause. Legionella infection causes 2 distinct clinical presentations: Legionnaires’ disease, an atypical pneumonia, and Pontiac fever, a mild upper respiratory infection with nonspecific influenza-like illness. Although respiratory sample culture is considered the gold standard for legionellosis diagnosis, it requires expertise and several days of incubation on complex media to perform. So, most diagnoses rely on commercial urinary antigen testing, due to the ease of specimen collection, same-day result, low cost, and simplicity of analysis.

However, this test has several limitations, including missing non-Lp1 infections (at least 21% of cases remain undiagnosed), other serogroups (L pneumophila), and nonserogroups (L micdadei, L bozemanii, and L longbeachae). Therefore, the overreliance on urinary antigen testing has created a diagnostic gap for Legionnaires’ disease caused by nonserogroup 1 L pneumophila and other species. In Europe, urinary antigen testing represents 82% to 90% of diagnostic tools used for Legionnaires’ disease confirmation. More than a decade ago, nucleic acid amplification tests (NAATs) that used real-time PCR were introduced for legionellosis diagnosis and now are increasingly accessible to laboratories on a moderate budget. Unlike urinary antigen testing, NAATs can be developed to target Lp1, other serogroups, and other species and has a significantly lower turnaround time than culture with specificities close to 100% and sensitivities better than urinary antigen testing and equal or greater to that of culture. Therefore, this multicenter, retrospective study investigated the risk of missing a diagnosis of legionellosis by relying on urinary antigen testing solely.

Researchers conducted a retrospective investigation of 14 Belgian hospital laboratories that introduced an in-house or a commercial real-time PCR method as part of a first-line testing algorithm for severe respiratory tract infections and compared this with urinary antigen testing results. A total of 71 routinely obtained results from patients with detectable Legionella DNA in a respiratory sample were included. Additionally, a primary “reverse” study was performed that included all patients who had a positive result from urinary antigen testing, and PCR was performed on both an upper and a lower respiratory tract sample within the same episode.


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Results showed that molecular Legionella detection had a higher sensitivity in lower respiratory tract specimens compared with upper respiratory tract specimens. In total, 20 (44.4%) of urinary antigen testing results were determined to be false negative and were reclassified as legionellosis based on PCR findings; more specifically, 15 were classified as Legionnaires’ disease and 5 were classified as Pontiac fever. Without the syndromic approach, a total of 26 (39.4%) diagnoses would have most likely been delayed or missed, as urinary antigen testing or specific molecular testing for Legionella was not clinician requested.

Overall, the study authors concluded that, “PCR is the only analysis method making it possible to search for Lp1 as well as non-serogroup 1 and, when targeted, non-pneumophila Legionella species within a clinically relevant and adequate time frame.”

Reference

Muyldermans A, Descheemaeker P, Boel A, Desmet S, Van Gasse N, Reynders M; on behalf of the National Expert Committee on Infectious Serology. What is the risk of missing legionellosis relying on urinary antigen testing solely? A retrospective Belgian multicenter study [published online December 14, 2019]. Eur J Clin Microbiol Infect Dis. doi:10.1007/s10096-019-03785-8