Potential false-positive urine Legionella enzyme immunoassay test results

The objective of this study was to identify potential false-positive urine Legionella pneumophila (Legionella) enzyme immunoassay test results. A total of 107 consecutive patients with positive EIA tests were retrospectively analyzed over a 34-month period. Concurrent blood, urine, and sputum cultures, as well as chest radiographic findings, were reviewed in these patients. Twenty patients (19%) had no radiographic evidence of pulmonary disease despite a positive EIA test. In those 20 patients, 14 also had growth of non-Legionella bacteria. Of patients with an infiltrate or opacity on chest imaging, only 27 had Legionella sputum cultures obtained, with Legionella culture growth occurring in 7 (26%). Nine other patients had negative Legionella sputum cultures but the growth of another pathogenic organism in blood, sputum, and/or urine cultures. Pseudomonas aeruginosa was the most common organism isolated, found in 20% of patients in the entire cohort. Twenty-five patients (23%) were characterized as having probable false-positive Legionella urinary antigen EIA testing, and an additional 17 patients (16%) were characterized as having possible false-positive Legionella EIA tests. Our findings suggest that urine Legionella EIA tests may lead to a substantial number of cases being misdiagnosed as Legionaries’ disease in patients with non-Legionella bacterial colonization or infection.