Question
Does the imperfect specificity of a N. gonorrhoeae NAAT lead to an unacceptably high percentage of false-positive results when used to screen individuals from a population in which there is a low prevalence of gonorrhea? 

Design
This report describes 5 false-positive gonorrhea test results related to use of a US Federal Drug Administration-approved nucleic acid amplification test (NAAT).

Participants
The cases of 5 women, aged 19 to 44 years, in long-term monogamous relationships, who presented to the Hawaii State Department of Health STD clinic with unanticipated positive results for N. gonorrhoeae by the Cobas Amplicor CT/NG test (Roche Diagnostics) were investigated.

Description of Tests and Diagnostic Standard
A false-positive case was defined as a patient who had an initial positive N. gonorrhoeae test whose result was negative on retesting using a different NAAT and culture, who did not receive appropriate antimicrobial therapy (or for whom there was not adequate time between receipt of appropriate antimicrobial therapy and retesting to account for a negative test result), and who had a low prior probability of infection with N. gonorrhoeae. 

Main Outcome Measures
The number of cases with a false positive N. gonorrhoeae Cobas Amplicor result was determined and the positive predictive value (PPV) calculated.  

Main Results
Genital specimens from all five women were initially tested at the same major private diagnostic laboratory by the Cobas Amplicor CT/NG test.  None of the 5 case subjects or 4 available partners had sexual histories or physical examination findings suggestive of N. gonorrhoeae infection and all had negative results of N. gonorrhoeae tests using another NAAT and culture. The time from private clinician examination to the Hawaii Department of Health clinic visit for retesting was 5 to 8 days for these 5 cases.  There were no significant detectable weekly variations in the number of N. gonorrhoeae or C. trachomatis tests performed or in the number of positive test results.  The proportion of positive test results was 1.06% (177 of 16641 tests) for N. gonorrhoeae.  Using published data on the Cobas Amplicor CT/NG test performance characteristics for detection of N. gonorrhoeae (sensitivity, 98.2%, specificity, 99.5%), the positive predictive value was calculated as 60% for a positive N. gonorrhoeae test result.  

Authors’ Conclusions
It is imperative that physicians obtain sexual histories from the patients they test so that they can apply clinical judgement to the interpretation of a positive screening test result.  False-positive test results should be considered when patients have unanticipated positive results of screening tests for N. gonorrhoeae or C. trachomatis, especially in cases in which the sexual history data or clinical findings do not support the laboratory results. Retesting using a different NAAT or culture to confirm a positive result is recommended in situations in which a positive test result is not supported by sexual history data or clinical findings.

Source of funding:  None given.

For correspondence:  Alan R. Katz, Department of Public Health Sciences and Epidemiology, John A. Burns School of Medicine, University of Hawaii, Biomedical Sciences Bldg., Rm. D104M, 1960 East-West Rd., Honolulu, HI 96822.  E-mail address:  katz@hawaii.edu.