Prospective evaluation of the Gen-Probe assay for detection of Legionellae in respiratory specimens

A prospective evaluation of a DNA probe assay for detection of Legionella species was performed on 427 consecutive respiratory specimens submitted over an 18-month period. The Gen-Probe assay utilizing both low (≥4.0) and high (>7.0) ratio threshold values was compared to direct fluorescent antibody staining (DFA) as a predictor of isolation of Legionella on culture. The highest sensitivity (63 %) was obtained with the lower threshold ratio, but was not significantly different from the result obtained with a threshold ratio of >7.0 (50 %, p=0.722) or DFA results (44 %, p=0.479). The specificity of the DNA probe assay was improved with the high threshold (99 %) compared either to the low threshold ratio (95 %, p=0.0002) or DFA (97 %, p=0.055). When the DNA probe was compared to DFA and/or Legionella isolation on culture, a significantly lower specificity (97 % versus 99 %, p=0.0006) and higher sensitivity (74 % versus 37 %, p=0.013) was obtained with a threshold value of ≥4.0 than >7.0. Ten of 20 specimens with a DNA probe ratio between 4.0 and 7.0 were DFA positive, although only two were isolated on culture. The DFA assay and both probe threshold ratios have a high negative predictive value when compared to culture. However, only the threshold ratio of >7.0 has a sufficiently high positive predictive value to be useful alone. Although the DNA probe appears to be a practical alternative to DFA testing for the rapid diagnosis of Legionella infections, false-negative results emphasize the importance of obtaining several specimens for testing, and confirm the fundamental role of culture in the diagnosis of Legionella infections.