Fifer & Ison (STIs) express concern over the use of the “dual” nucleic acid amplification tests (NAATs) for the detection of chlamydia and gonorrhoea in the context of chlamydia screening in the UK. Additional testing for gonorrhoea, when the real target is chlamydia, does not necessarily confer an additional net benefit. This is because even a high specificity test such as Cobas 4800 (Perry & Corden (STIs); Rockett & Limnios (STIs)) will generate a high proportion of false positives when the infection tested for has extremely low prevalence, as in the case of gonorrhoea in the general population. And the potential disbenefit of the additional test in terms of the psychological impact, and the impact on relationships, of false positive diagnoses could easily outweigh the medical benefit represented by the diagnoses which are accurate (Dixon-Woods & Shukla (STIs); McCaffery & Wardle (STIs)).
The potential impact of the adoption of the dual NAAT as a stand-alone test – if not confirmed by further testing using either a second NAAT or else culture – is illustrated by a recent Australian study published in the Medical Journal of Australia (MJA). Chow & Fairley perform a retrospective analysis of insurance and notification data from Melbourne over the years 2008-2013. They seek to demonstrate that the apparent rise in identified gonorrhoea cases amongst the general female – non-indigenous – population (from 98 to 343) is at least partly an “artefact” of the growing employment by laboratories of the dual NAAT. They do this by eliminating the alternative possibility of a genuine increase in gonorrhoea in the general population. To this purpose they use their data to investigate changes in the proportion of positive dual NAAT gonorrhoea diagnoses to the number of dual NAAT test ordered, over the period during which dual NAATs were being introduced. They also investigate rates of positive gonorrhoea diagnoses over this period at a “sentinel” clinic in Victoria where culture alone was used as a means ofgGonorrhea diagnosis. They find that the proportion of positive dual NAAT diagnoses in Victoria remained relatively constant over time (around 0.2-0.3%), as did the proportion of positive culture diagnoses at the Melbourne clinic (around 0.4-0.6%). Of 25 untreated women who had a positive NAAT result for gonorrhoea and were referred to the Melbourne clinic, only 10/25 were confirmed by culture. The authors comment that this is in line with what might be expected in the light of the published specificity of the various NAAT tests employed.
C&F recommend that laboratories suppress gonorrhoea diagnoses from the dual NAATs. An MJA editorial in the same issue questions the feasibility of this. Instead, the editors propose that the NAAT should, in the case of Gonorrhoea, be used as either a triage, with positive diagnoses confirmed by culture, or as an add-on where high prevalence populations are first tested by culture. They also consider the possibility of confirming the initial NAAT with a NAAT using a different target. However, they come down in favour of retaining culture in the diagnostic pathway on account of its value as a means of assessing resistance. They also question whether even the double NAAT would guarantee adequate predictive value in very low prevalence populations.
Evidently, further studies are required.