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Journal of Clinical Microbiology, September 2001, p. 3092-3098, Vol. 39, No. 9
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.9.3092-3098.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Enhancing the Specificity of the COBAS AMPLICOR CT/NG Test for Neisseria gonorrhoeae by Retesting Specimens with Equivocal Results

Barbara Van Der Pol,1 David H. Martin,2,3 Julius Schachter,4 Thomas C. Quinn,5,6 Charlotte A. Gaydos,6 Robert B. Jones,1 Kimberly Crotchfelt,6 Jeanne Moncada,4 D. Jungkind,7 Buffy Turner,8 Cynthia Peyton,8 James F. Kelly,9 Judith B. Weiss,9 and Maurice Rosenstraus9,*

Indiana University School of Medicine, Indianapolis, Indiana1; Louisiana State University2 and City of New Orleans' Delgado Clinic,3 New Orleans, Louisiana; University of California---San Francisco, San Francisco,4 and Roche Molecular Systems, Pleasanton,9 California; National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda,5 and Johns Hopkins University, Baltimore,6 Maryland; Thomas Jefferson University Hospital, Pittsburgh, Pennsylvania7; and University of Texas Medical Branch, Galveston, Texas8

Received 7 December 2000/Returned for modification 28 April 2001/Accepted 20 June 2001

The COBAS AMPLICOR CT/NG test for Neisseria gonorrhoeae cross-reacts with certain strains of nonpathogenic Neisseria species. In some strains, the target sequence is identical to that of N. gonorrhoeae, whereas other strains have a small number of mismatches within the regions recognized by the primers or probe used in the COBAS AMPLICOR NG test. These cross-reactive strains are occasionally present in urogenital specimens, causing false-positive results in the COBAS AMPLICOR NG test. Analysis of the data generated in a large multicenter clinical trial showed that 2.9% of the specimens gave signals between A660s of 0.2 and 3.5 but that one-half of these equivocal specimens did not contain N. gonorrhoeae. Most of these equivocal specimens were correctly classified as true positive or true negative by retesting in duplicate and defining a PCR-positive result as two of three results with an A660 of >= 2.0. If specimens had been classified as positive or negative based on a single test result using a cutoff of an A660 of 0.2, specificity would have ranged from 96.2 to 98.9% depending on specimen type, sex, and presence of symptoms. By employing the equivocal zone-retesting algorithm, specificity increased to 98.6 to 99.9% with little effect (0.1 to 4.9% decrease) on sensitivity in most specimen types, enabling the test to achieve a positive predictive value of at least 90% in populations with a prevalence of 4% or higher. In lower-prevalence populations, the test could be used to screen for presumptive infections that would have to be confirmed by an independent test.

* Corresponding author. Mailing address: Roche Molecular Systems, 1080 U.S. Highway 202 South, Somerville, NJ 08876. Phone: (908) 253-7463. Fax: (908) 253-3318. E-mail: maurice.rosenstraus{at}roche.com.

Journal of Clinical Microbiology, September 2001, p. 3092-3098, Vol. 39, No. 9
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.9.3092-3098.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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