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 (greater than or equal to 4.0) and high (greater than 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 greater than 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 = 0.002) 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 greater than or equal to 4.0 than greater 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 greater than 7.0 has a sufficiently high positive predictive value to be useful alone.(ABSTRACT TRUNCATED AT 250 WORDS)