Second tests for Chlamydia trachomatis and Neisseria gonorrhoeae

Sue Skidmore, Dr ,
, ,

Other Contributors:

August 02, 2012

The paper by Hopkins et al suggests that repeat testing for C trachomatis (CT) and N gonorrhoeae (NG) is unnecessary.1 This issue has long been debated 2,3,4 and currently, with many laboratories having to reduce costs, the view put forward in this paper seems attractive. We would, however, like to make the following points. Repeat testing for CT using the same platform is not recommended for the purpose of confirmation. Confirmation must be by a second assay with similar performance power to the initial assay. Repeat testing was initially introduced to help identify any mismatch errors due to the significant manual intervention required in the early testing platforms. Although advances in technology have reduced this, repeat testing does also serve to identify issues with contamination which, because of the high volume of samples being tested, can be significant. This remains a consideration and so, if repeat testing is stopped, robust IQA measures must be put in place together with a strict cleaning and decontamination regimen. In addition, many young people may be happy to receive unnecessary antibiotic treatment, as stated in the paper, but we are constantly being warned about the overuse of antibiotics and this must be taken into account. For NG testing, BASHH guidelines ( ) state that reactive results should be confirmed to ensure that PPV is >90% and it is essential that the confirmatory test is robust with respect to target selection as well as the usual statistical performance characteristics as per CT. The best strategy to use for an STI is to employ a sensitive test which ensures that no cases are missed followed by confirmation to verify the true result. This paper looked at Genitourinary Medicine patients and established the PPV for this group although there were only 2 patients with equivocal results to investigate. In laboratories using dual testing on samples from the NCSP programme where the prevalence of infection may be much lower, it would be prudent to use the suggested confirmatory algorithm unless extensive validation work has shown that it is not required. We agree that a PPV of >95% for screening should be the standard for both infections as the authors suggest.

1. Hopkins MJ, Smith G, Hart I et al. Screening tests for Chlamydia trachomatis or Neisseria gonorrhoeae using the cobas 4800 PCR system do not require a second test to confirm: an audit of patients issued with equivocal results at a sexual health clinic in the Northwest of England, UK. Sex Transm Infect 2012

2. Schachter J, Chernesky MA. Routine Confirmation of Positive Nucleic Acid Amplification Test Results for Neisseria gonorrhoeae is Not Necessary. J Clin Microbiol 2012; 50:208

3. Tabrizi SN, Hjelmevoll SO, Garland SM et al. Reply to above. J Clin Microbiol 2012; 50:209-10

4. Schachter J, Chow JM, Howard H et al. Detection of Chlamydia trachomatis by nucleic acid amplification testing: our evaluation suggests that CDC-recommended approaches for confirmatory testing are ill-advised. J Clin Microbiol 2006; 44: 2512-7

Conflict of Interest:

None declared

Conflict of Interest

None declared