Literature review > Issue_2 > Review West et al. 

This is a well-written paper that gives much needed information regarding the performance and utility of simple rapid tests that can be used in field settings. This study was conducted as part of a reproductive health survey on women aged 15-54 in the Gambia.

Non-treponemal specific tests, such as the rapid plasma reagin (RPR) test, have been tremendously important tools for the control of syphilis. Since antibody response to non-treponemal antigens rise in response to infection and decline with successful treatment, these tests remain the only tools for the diagnosis of active infection to date.

Because of its simplicity, the RPR test has been used in settings with no or limited access to laboratory services. Although easy to perform, the interpretation of the test result is subjective and the interpretation is especially difficult when the test result is in the "grey zone" between a positive and a negative result. In addition, as the authors pointed out, when the test is performed in field conditions, the test results can be affected by other problems such as high heat, dust, poor light and the reaction drying up before it can be read. In this study, even though the tests were performed by the same well-trained technicians in both the laboratory and the field, 12 of 47 sera positive by the lab RPR test were interpreted as negative in the field, and 41 of 1248 sera negative by the lab result were read as positive in the field. This would have led to the over-treatment of 41 women with false positive RPR results, but more importantly 12 women with syphilis would have been missed, with serious consequences if they were pregnant. Imagine the scenario where these tests are performed under similar conditions but by relatively untrained personnel with limited or no quality control. Congenital syphilis is still a public health problem in much of the developing world and as the authors pointed out, the unreliability of a negative screening test result and missed opportunity for treatment is a serious concern. Are there any new tools for syphilis screening?

In recent years, more than 20 rapid treponemal tests have become commercially available. These tests can be used with whole blood, require minimal training, no equipment or special storage conditions, and give a visual readout in 8-20 minutes. Several rapid tests, with sensitivities and specificities of 85-98% and 92-98% respectively compared to standard treponemal assays, are now available through the WHO Bulk Procurement Scheme at negotiated prices between US$0.47 to $1.40. A report of the performance and operational characteristics of 6 rapid tests evaluated by WHO is available from www.who.int/std_diagnostics

Although this appears to be good news for increasing access for syphilis screening to areas where such laboratory services are either not available or RPR is performed poorly, a limitation for using treponemal tests as screening tests in high prevalence settings is that treponemal antibodies are retained for years and hence there may be a high background of treponemal antibodies from past treated infections. The authors evaluated the performance of one of these rapid tests and calculated the PPV and negative predictive values for different prevalence settings. The sensitivity and specificity of the treponemal test they evaluated was 75% and 95% respectively. Hence the treponemal test evaluated in this paper has the advantages of ease of use and interpretation but offers no better predictive values than the field RPR test. In the report of the WHO evaluation, rapid tests with sensitivities of 98% compared to standard laboratory treponemal tests tend to have specificities lower than 95% and vice versa. Given the serious consequence of missed diagnosis, it is important to use the most sensitive test for screening, but as the authors rightly pointed out, there are also serious socio-psychological consequences for false positive tests. In the absence of a good non-treponemal test, it may be possible to use a combination of rapid tests--tests with high sensitivity for screening and tests with high specificity for confirmation.

The ideal test for syphilis screening is of course one that combines the advantages of non-treponemal and treponemal tests, i.e. has high sensitivity and specificity, can distinguish between active and past treated infection, use whole blood and has all the operational characteristics of rapid tests. Such a test can also be used for test of cure. The major barriers to the development of such a test include the perceived lack of return on investment on the part of the diagnostic industry for developing a new syphilis test and technical problems in making non-trepnemal antigens adhere to the same solid matrix as treponemal tests. The STD Diagnostics Initiative is working with public and private sector developers to provide incentives downstream of test development in this area such as the provision of laboratory and field sites for test evaluation, use of WHO evaluation data for regulatory approval and, if the evaluation results are promising, access to a wide market through the WHO Bulk Procurement Scheme (the scheme is opened to 192 member states of the UN and UN agencies).

Considering the number of rapid syphilis tests being sold all over the world, there have been very few papers published on their performance and utility. We need more papers like this one that evaluate the performance of different syphilis tests in primary health care settings and examine their utility in terms of achieving better public health outcomes.

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