Evaluation of real-time PCR for the early detection of Legionella pneumophila DNA in serum samples

Legionella pneumonia can be difficult to diagnose. Existing laboratory tests all have shortcomings, especially in the ability to diagnose Legionnaires' disease (LD) at an early stage of the disease in a specimen that is readily obtainable. The aim of this study was to assess the performance of PCR as a rapid diagnostic method and to compare the results of different PCR assays of serum samples from patients with LD. Samples included 151 serum samples from 68 patients with proven LID and 60 serum samples from 36 patients with respiratory tract infections other than Legionella. PCR assays were based on the 5S rRNA gene, 16S rRNA gene and the mip gene. The samples from patients with infections caused by pathogens other than Legionella all tested negative in PCR. Among the patients with proven LD 54.4% (37/68) tested positive in 5S rRNA PCR, 52.9 % (36/68) in mip gene PCR and 30.9 % (21/68) in 16S rRNA PCIR in the first available serum sample. The association between threshold cycle value in 5S PCIR positive serum samples (n = 49) and C-reactive protein value was determined, and showed a strong negative correlation (Pearson correlation coefficient r= -0.63, P < 0.0001). In addition to existing tests for the diagnosis of LID, detection of Legionella DNA in serum could be a useful tool for early diagnosis of LID caused by any Legionella species and serogroup, and has the potential to provide a diagnosis in a time frame that could affect initial infection management.