Opportunistic screening for Chlamydia

Editor—The strand displacement amplification test used by Moens et al for Chlamydia trachomatis screening produced equivocal results in 11.2% of samples.¹ Such a high rate is unacceptable.

A result could be termed equivocal if it is borderline or if a sample gives discrepant results on repeat testing. In addition, this assay has an internal control, a negative result being invalid if the internal control is inhibited. Differentiation between these potential causes is important—borderline and non-repeatable positive results could be due to small amounts of organisms or technical problems such as cross contamination; inhibitors may be prevented by cold chain transportation of urine or by using the urine processing pouch supplied by the manufacturer.²

We have extensive experience with the strand displacement amplification test and have not experienced such a problem with equivocal results. Of 24 130 samples tested, 21 605 (89.5%) were negative while 2224 (9.2%) were repeatable positive results. Only 72 samples gave non-repeatable positive results (prevalence 0.3%, reproducibility rate 96%); 229 samples (0.9%) had evidence of inhibitors. Thus, only 1.2% of the samples gave no definitive positive or negative result.

Although treating all equivocal results as positive is a pragmatic approach, action based on poor quality results will lead to misleading reports, inappropriate labelling of patients as having sexually transmitted infections, unnecessary partner notification, overtreatment with antibiotics, inaccurate statistics, and incorrect epidemiological analysis. Active microbiological input is essential during the planning of such programmes to optimise sample collection and transportation as well as the interpretation of results.