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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
  1. Mark J Hopkins1,
  2. Godfrey Smith2,
  3. Ian J Hart1,
  4. Fath Alloba3
  1. 1Liverpool Specialist Virology Centre, Royal Liverpool University Hospital, Liverpool, UK
  2. 2Department of Medical Microbiology, Royal Liverpool University Hospital, Liverpool, UK
  3. 3Department of Genitourinary Medicine, Royal Liverpool University Hospital, Liverpool, UK
  1. Correspondence to Dr Mark J Hopkins, Liverpool Specialist Virology Centre, Royal Liverpool University, Hospital, Liverpool L7 8XP, UK; m.hopkins{at}


Objectives To assess the clinical utility of supplementary PCRs following a positive cobas 4800 CT/NG PCR screening test result.

Methods Laboratory reports, for Chlamydia trachomatis or Neisseria gonorrhoeae, issued to genitourinary medicine patients between April 2010 and April 2011 were reviewed retrospectively. Positive reports were routinely confirmed by supplementary PCRs and N gonorrhoeae culture. Clinical records of patients with unconfirmed positive (equivocal) reports were retrieved to determine if the infection was confirmed by a second sample obtained at patient recall and the impact of this process on antibiotic management.

Results Over 15 000 patients were tested during the study period. The prevalence of chlamydia and gonorrhoea was 972 (5.75%) and 76 (0.50%), respectively. A further 78 chlamydia and 2 gonorrhoea equivocal reports were issued. Only 56 (72%) patients with an equivocal chlamydia report returned to the clinic, and of these, only 41 (73%) gave a second sample to retest. Positive predictive value (PPV) of the PCR screening test was calculated at 98.0% and 97.5% for detection of chlamydia infection from urine and rectal swabs, respectively. Most patients accepted antibiotic treatment before their infection status had been confirmed. Prevalence of gonorrhoea infection was low but the PPV of the screening PCR in urine specimens remained high (98.75%).

Conclusions Equivocal reports introduce delays to patient management, while the risk of unnecessary antibiotic therapy appears acceptable to most patients. The cobas 4800 CT/NG PCR screening assay can achieve UK testing standards (PPV >90%) for chlamydia, and low prevalence gonorrhoea in urine without supplementary tests. A patient-led confirmation algorithm is proposed.

  • Chlamydia infections
  • diagnostics
  • gonorrhea
  • PCR
  • guideline development
  • virology clinical
  • virology HIV
  • virology HPV/HSV
  • virology general laboratory

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  • Ethics approval Audit approved by the Royal Liverpool University Hospital Clinical Audit Working Group (Ref: 3993-11/12).

  • Provenance and peer review Not commissioned; externally peer reviewed.