Neisseria species identification assay for the confirmation of Neisseria gonorrhoeae-positive results of the COBAS Amplicor PCR

Abstract

Screening assays for Neisseria gonorrhoeae exhibit low positive predictive values, particularly in low-prevalence populations. A new real-time PCR assay that detects and identifies individual Neisseria spp. using melt curve analysis was compared to two previously published supplementary assays. NsppID, a 16S rRNA real-time PCR/melt curve assay developed to distinguish N. gonorrhoeae from other Neisseria spp., was compared to real-time PCR assays targeting genes reportedly specific for N. gonorrhoeae, the cppB gene and the porA pseudogene. A total of 408 clinical specimens (324 female endocervical swabs and 84 male urine or urogenital swab specimens) were screened using the COBAS Amplicor assay for Chlamydia trachomatis and N. gonorrhoeae (CT/NG) (Roche Diagnostics, Indianapolis, IN) followed by confirmatory testing via real-time PCR. The NsppID assay detected Neisseria spp. in 150/181 COBAS-positive specimens (82%), including six dual infections, and identified N. gonorrhoeae in 102 (56%) specimens. Sixty-nine of 181 (38%) specimens were positive for N. gonorrhoeae by porA pseudogene, and 115/181 (64%) were positive for cppB. However, cppB was also positive in 15% of COBAS-negative specimens, more than either NsppID (4%) or porA pseudogene (2%) assays. The porA pseudogene assay had the highest specificity for both genders but the lowest sensitivity, especially in female specimens. NsppID had a slightly lower specificity but greater sensitivity and overall accuracy. The least desirable confirmatory assay was cppB, due to poor specificity. The NsppID assay is an accurate confirmatory assay for N. gonorrhoeae detection. In addition, the NsppID assay can identify the non-N. gonorrhoeae species responsible for the majority of false-positive results from the COBAS Amplicor CT/NG assay.

FIG. 1.

Sequence comparisons between 10 Neisseria spp. encompassed by the FRET probes used in the NsppID assay. The underlined sequences are exact matches to the probe sequences and therefore have the highest theoretical T_m. The sequence numbering listed for the two probes is from GenBank accession no. X07714.

FIG. 2.

Melt curve analysis in the NsppID assay using the ATCC controls, showing species-specific T_m peaks.