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How likely is environmental or patient cross-contamination of Chlamydia trachomatis DNA to lead to false positive results in patients attending our clinic?
  1. Sum Yee Chan1,
  2. Sophie Jose2,
  3. Rebecca King1,
  4. Mark R Pakianathan1,
  5. Caroline Sabin2,
  6. S Tariq Sadiq3,
  7. Phillip E Hay1,
  8. Tim Planche4
  1. 1The Courtyard Clinic, St Georges Hospital, London, UK
  2. 2Department of Infection and Population Health, University College London, London, UK
  3. 3Department of Cellular and Molecular Medicine, St Georges University of London, London, UK
  4. 4Centre for Infection and Immunity, St Georges University of London, London, UK
  1. Correspondence to Dr Sum Yee Chan, The Courtyard Clinic, Blackshaw Road, Tooting, London SW17 0QT, UK; sumyeeyorkies{at}


Objectives Environmental contamination with DNA from Chlamydia trachomatis (CT) has previously been found in Genitourinary Medicine (GUM) clinics. There are no known cases of cross-contamination of clinical samples and no known nosocomial infections. We investigated whether diagnostic samples could become contaminated from the environment by running dummy sample and carrying out a patient-throughput analysis. A total of 29 748 patients attended clinics over a year. Of these, 2860 (9.6%) had a positive Chlamydia test result.

Method (1) A run of dummy samples (60 urine samples and 10 swabs) were processed as normal clinic specimens. (2) Patient-throughput analysis: Patient numbers attending the GUM clinic on a given day was categorised as low, moderate or high. χ2 Tests were used to look for associations between categorical variables and Chlamydia test positivity. A Poisson regression model was fitted to look at the effect of the number of people in the clinic on the number of positive results in a given day. As some clinics were only run on certain days of the week, a sensitivity analysis was later performed with attendances at non-daily clinics removed.

Results All dummy samples tested negative and we did not find evidence of an association between daily Chlamydia positivity and clinic attendance.

Conclusions It is unlikely that environmental or cross-contamination of CT has lead to significant numbers of false positive results. Laboratories check for possible cross-contamination routinely. The extension of this simple routine practice to all clinical areas could provide quality assurance, improving confidence in the results in clinics.

  • Chlamydia Trachomatis
  • Infection Control
  • Bacterial Infection
  • Chlamydia Infection
  • Genitourinary Medicine

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