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

Abstract

SUMMARY: The efficiency of the direct detection ofMycoplasma pneumoniaein 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 pneumoniaeRapid Diagnostic System’, Gen-Probe, San Diego, California).Both assays showed excellent specificity against a range of mycoplasma species suspended in negative nasopharyngeal aspirates; onlyM. pneumoniaeandM. genitaliumreacted. In experiments with graded doses of viableM. pneumoniaecells 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 × 10⁵genomes) and 2·5 × 10⁴c.f.u./ml (4 × 10⁶genomes); 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 forM. pneumoniaeand 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.