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Journal of Clinical Microbiology, November 2002, p. 4056-4059, Vol. 40, No. 11
0095-1137/02/$04.00+0     DOI: 10.1128/JCM.40.11.4056-4059.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Confirmation by 16S rRNA PCR of the COBAS AMPLICOR CT/NG Test for Diagnosis of Neisseria gonorrhoeae Infection in a Low-Prevalence Population

David J. Diemert,* Michael D. Libman, and Pierre Lebel

Department of Microbiology, Montreal General Hospital, McGill University Health Centre, Montreal, Canada

Received 7 May 2002/ Returned for modification 14 June 2002/ Accepted 15 August 2002

The COBAS AMPLICOR CT/NG test is widely used for the diagnosis of Neisseria gonorrhoeae infection using genital swabs or urine samples. Although highly specific, cross-reactivity occurs with some nonpathogenic strains of Neisseria and Lactobacillus species. In low-prevalence populations, even highly specific assays may require confirmatory testing of positive results. We assessed the positive predictive value (PPV) of this test in a low-prevalence (0.5%) setting. Genital and urine specimens testing positive using the COBAS AMPLICOR NG test were retested using an investigational 16S rRNA PCR assay. Additionally, 737 specimens were tested in parallel by both culture and the above PCR protocol. Of 9,772 specimens tested in-house, 168 were positive by the AMPLICOR test; in addition, 62 AMPLICOR-positive specimens were referred to our laboratory for confirmatory testing, yielding 230 positive specimens. Of these, 72 were confirmed positive by 16S rRNA PCR, yielding a specificity of 98.7% and a PPV of 31.3%. Specificity was similar for all specimen types, whereas PPV varied with prevalence: specimens from males, females, urine specimens, and genital swabs had PPVs of 70.8, 13.3, 51.9, and 20.1%, respectively. The PPV was higher when the initial AMPLICOR optical density (OD) was >=3.5 versus initial and repeat OD readings in an equivocal zone of >=0.2 to <3.5 (65.1 versus 10.1%; P < 0.001). On repeat testing of specimens with ODs in the equivocal zone, 54 gave ODs of >=0.2 and <2.0, 35 gave ODs of >=2.0 and <3.5, and 12 gave ODs of >=3.5, with 3.7, 20, and 33.3% confirmed positive, respectively (P = 0.004). Comparing PCR to culture as the "gold standard," specificity increased from 96.8 to 99.9% when 16S rRNA PCR was performed on specimens positive by the COBAS AMPLICOR NG test. Confirmatory testing with a more specific method such as 16S rRNA PCR should be considered in low-prevalence populations, especially for specimens with an OD in the equivocal zone.

* Corresponding author. Present address: Malaria Vaccine Development Unit, National Institute of Allergy and Infectious Diseases, Twinbrook I, Room 1123, 5640 Fishers Ln., Rockville, MD 20852. Phone: (301) 443-3915. Fax: (301) 480-1962. E-mail: ddiemert{at}niaid.nih.gov.

Journal of Clinical Microbiology, November 2002, p. 4056-4059, Vol. 40, No. 11
0095-1137/02/$04.00+0     DOI: 10.1128/JCM.40.11.4056-4059.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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