In the recent STI testing report (April 2009), I found one paragraph quite confusing which made me go to the
article to clarify what you meant. In the report you state the following;
“Currently, approximately only 9% of all Chlamydia tests performed in New Zealand return a positive
result. A study in London was able to demonstrate that by using a risk assessment strategy based on testing those under
25 who had two or more sexual partners in the past year, they were able to increase the yield of positive results to 87%.”
This seemed to be saying that by using this strategy you could get 87% of your tests positive - a remarkable number
indeed but alas not so. The 87% refers to detecting 87% of the positives in the screened population. To do this they had
to screen 49% of their sample of women. The actual yield rate would have been about 20/445 or about 5%.
Am I right or have I misread this?
Dr Michael Brewer, GP, Motueka
The short answer is that you are right. This paragraph has confused the number of positive results with the sensitivity
of a particular screening strategy. The best way of clarifying this may be to take a closer look at the study itself
(the long answer).
The study by Grun et al had three objectives:
- To estimate the prevalence of Chlamydia trachomatis in asymptomatic women attending general practice
- To assess the potential of the ligase chain reaction as a screening tool
- To evaluate selective screening criteria
The third objective was the focus of the bpacnz report because asymptomatic infection is unlikely to be
detected without a screening programme. But given the relatively low prevalence of infection, it is more appropriate
to consider targeted screening than universal testing.
When considering targeted screening strategies Grun et al noted that younger age and multiple partners were associated
with infection. They tested possible combinations of age and number of partners to identify a strategy that detected
the greatest number of infections for the least number of people tested.
The study group of 879 women aged 18�35 years were all tested and 23 Chlamydia infections were detected.
If only women aged 25 years or less had been screened, 17 of 23 infections (74%) would have been detected by testing
approximately 35% of the study population
If only women aged 29 years or less had been screened, 20 of 23 infections (87%) would have been detected by testing
approximately 67% of the study population
If only women aged 25 years or less and all women who had had two or more partners in the past year had been screened,
20 of 23 infections (87%) would have been detected by testing approximately 49% of the study population
While no targeted screening strategy detected all the cases of infection, testing women aged 25 years or less and all
women who had had two or more partners in the past year detected the greatest number of infections for the least number
of people tested.
- Grun L, Tassano-Smith J, Carder C, et al. Comparison of two methods of screening for genital chlamydial infection
in women attending in general practice: cross sectional survey. BMJ 1997;315:226-230.