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Point of care testing

The test-based diagnosis of asymptomatic chlamydial infection usually involves sending a clinical specimen to a laboratory elsewhere with considerable delay until the results become available. Inevitably some patients lose patience with this process and fail to attend the follow up appointment. Rapid point of care tests are available for clinic use and most patients would probably be prepared to wait 40 - 50 minutes for a test result in order to receive immediate treatment or reassurance, particularly if necessary specimens were taken immediately on patient reception in the clinic. Until now, lab-processed tests have generally had much greater sensitivity than rapid, point of care tests. Gift et al., 1999 studied whether the less sensitive but more convenient rapid tests might actually lead to the treatment of more patients and thus be more cost-effective. They used a decision analysis framework to compare the BioStar Chlamydia OIA rapid test with two lab-based tests: cell culture and a nucleic acid amplification based (PCR) test. It was assumed that all women in the model would be screened. Variables included in the analysis were the prevalence of infection, test sensitivity and specificity, the probability of developing pelvic inflammatory disease after treated and untreated chlamydial infection, and the likelihood that patients would wait for rapid test results or return to the facility for treatment. It was concluded that the rapid test treated more cases of infection than the PCR alone if the return rate was less than 65%. [This is an important point which warrants further investigation]. A two-test algorithm of the rapid test followed by a PCR test on those initially testing negative identified and treated the greatest number of chlamydial infections. It was considered the most cost-effective strategy at a prevalence of infection above 9%. However this finding was sensitive to the cost estimates for pelvic inflammatory disease. They concluded that in settings where patient return for treatment is a problem, point-of-care tests might contribute significantly to the detection and treatment of chlamydial infections among asymptomatic women.

In a multi-centre study, Vickerman et al., 2003 used data from developing countries and also England to develop a mathematical model to explore how the sensitivity requirements for a rapid point of care test might differ in different clinical settings. Data used were from Benin, South Africa, Tanzania, and England. They found that the required sensitivity of a point of care test is low if either there is significant STI transmission during the delay in treatment for the gold standard laboratory based test and/or few women return for treatment. In the case of C. trachomatis, the required sensitivity of a point of care test is 50% (versus assumed gold standard test sensitivity of 90%) if either 55% return for treatment and there is no STI transmission, or 80% return for treatment and 50% of infected women infect their partner during the delay in treatment. Importantly, they concluded that a rapid point of care test of moderate sensitivity could lead to significantly more sexually transmitted infection averted than the more delayed gold standard test.

[Comment: This is an important concept and there are a surprising lack of studies of the issue. The writer is associated with a UK Department of Health funded, simple, automated, rapid test for use in clinics and primary care with equivalent sensitivity to conventional nucleic acid amplification-based tests. New generation improved rapid tests are also under development elsewhere. Such tests may have considerable impact on present-day screening strategies, shifting the focus of testing from delayed laboratory tests to rapid clinic-based tests].

[MEW] July 2004

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References

Gift, T. L., Pate, M. S., Hook, E. W. 3rd. &  Kassler, W. J. (1999). The rapid test paradox: when fewer cases detected lead to more cases treated: a decision analysis of tests for Chlamydia trachomatis. Sexually Transmitted Diseases 26, 232 - 240.

US CDC Guidelines on screening tests to detect C. trachomatis and N. gonorrhoeae. October 2002 [Important guideline. Page 6 includes useful comment on point of care tests]

Vickerman, P., Watts, C., Alary, M., Mabey, D. & Peeling, R. W. (2003). Sensitivity requirements for the point of care diagnosis of Chlamydia trachomatis and Neisseria gonorrhoeae in women. Sexually Transmitted Infections 79, 363 - 367.