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Bacterial STIs
P53 PCR screening tests for Chlamydia trachomatis or Neisseria gonorrhoeae do not require a second test to confirm: an audit of patients issued with equivocal results
  1. M Hopkins,
  2. G Smith,
  3. I J Hart,
  4. F Alloba
  1. Royal Liverpool University Hospital, Liverpool, UK


Background UK testing algorithms for chlamydia or gonorrhoea should have a positive predicative value (PPV) >90%. Repeat of testing of screen positive samples might be required to achieve this. Patients issued with unconfirmed positive (equivocal) results are recalled to clinic to submit another sample.

Objectives To assess the clinical utility of supplementary PCRs following a positive PCR screening test result.

Methods Laboratory reports for Chlamydia trachomatis or Neisseria gonorrhoeae issued to GUM 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 equivocal reports were retrieved to determine if infection was confirmed by a second sample on 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 clinic, and of these, only 41 (73%) gave a second sample to retest. 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 infection status had been confirmed. Prevalence of gonorrhoea infection was low but PPV of the screening PCR 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%) in extra-genital swabs and low prevalence gonorrhoea population without supplementary tests. A patient-led confirmation algorithm is proposed.

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