Laboratory diagnosis of Mycoplasma pneumoniae infection: 2. Comparison of methods for the direct detection of specific antigen or nucleic acid sequences in respiratory exudates

SUMMARY The efficiency of the direct detection of Mycoplasma pneumoniae in respiratory exudates by an antigen capture, indirect enzyme immunoassay (Ag-EIA), has been compared with its detection with a cDNA probe (‘Gen-Probe assay’) directed against the specific ribosomal RNA sequences of the organism (‘Mycoplasma pneumoniae Rapid Diagnostic System’, Gen-Probe, San Diego, California). Both assays showed excellent specificity against a range of mycoplasma species suspended in negative nasopharyngeal aspirates; only M. pneumoniae and M. genitalium reacted. In experiments with graded doses of viable M. pneumoniae cells suspended in negative nasopharyngeal aspirate, the Gen-Probe assay was more sensitive than Ag-EIA; detection limits were respectively 2 × 103 c.f.u./ml (3·2 × 105 genomes) and 2·5 × 104 c.f.u./ml (4 × 106 genomes); detection levels 10–100 times less sensitive than culture. The two assays were also tested on nasopharyngeal aspirates or sputum specimens from 90 patients with respiratory infection; 67 of these were culture-or seronegative for M. pneumoniae and 23 were culture-or seropositive. Ag-EIA detected 21 (91%) of the latter but the Gen-Probe assay detected only 5 (22%). Both assays were negative with the 67 culture-/sero-negatives; there were no Gen-Probe assay positive/Ag-EIA negatives. Overall, it is concluded that although Ag-EIA and the Gen-Probe assay are effective substitutes for culture as a diagnostic procedure, there is a significant problem with samples which are culture-negative and from patients who have good serological evidence of current infection. Possible reasons for the disparity between the two assays are advanced.