Detection of Legionella pneumophila DNA in Serum Samples from Patients with Legionnaires' Disease

Abstract

Diagnosis of Legionnaires’ disease (LD) in patients with pneumonia is based on phenotypic (culture, serologic testing, antigen detection in urine) and genotypic PCR methods. This chapter assesses the sensitivity and specificity of Legionella DNA detection using PCR on serum samples from patients with proven LD and patients with infections other than Legionella. Serum samples from two healthy volunteers were included as negative controls after every four samples. One patient was admitted to the emergency department of our hospital with proven LD caused by Legionella pneumophila serogroup 1 (urinary antigen test positive, culture positive, and sputum PCR positive). From this patient, consecutive serum samples were collected for Legionella-specific, real-time PCR. The results of PCR showed an increase in Ct values (corresponding with a logarithmic decrease of bacterial DNA) in the course of time and were found to mirror the clinical condition of the patient and c-reactive protein values during the acute stage of infection. Serologic methods to diagnose L. pneumophila infections are highly sensitive, but their utility is generally limited to epidemiologic studies due to the time lag needed to detect seroconversion. The advantages of PCR for the detection of L. pneumophila DNA in serum are evident; serum samples are readily obtainable and can be processed within a working day.