Rapid tests for syphilis

Diagnostic overview

Syphilis is one of many possible causes of genital ulcer disease (GUD). However, clinical diagnosis is of limited use since chancres may heal or can be atypical, and infected patients may also be asymptomatic. While the lesions produced during syphilis infection often have characteristic appearances, clinical studies in the field have shown that the accurate differentiation from lesions produced by other genital infections is inaccurate and difficult. Culture of Treponema pallidum is not possible and does not represent a diagnostic alternative.

Laboratory-based diagnostic tests

Dark-Field Microscopy

Dark-field microscopy (DFM) is a useful diagnostic technique when an active chancre or moist rashes such as condyloma latum (a specific type of rash that occurs in stage II syphilis) are present. However, the accuracy of DFM is dependent on a number of factors such as the experience of the microscopist, availability of good quality equipment, prompt examination of prepared slides, and the number of live treponemes in the lesion. This technique is not recommended for use with oral or anal lesions due to the presence of nonpathogenic, non-T. pallidum treponemes. It also is not recommended for evaluating dry sores. This test is used mainly to diagnose syphilis in primary, secondary, or early congenital syphilis.

Serology tests

Syphilis serology tests can largely be divided into two broad categories: treponemal and non-treponemal. Treponemal tests are those that look for a direct antigen from the T. pallidum or an antibody to it. These antibodies can remain in the bloodstream for many years following exposure to syphilis. Thus, a positive result for a treponemal test does not necessarily indicate active syphilis infection. Non-treponemal tests look for an incidental marker, usually a cardiolipin, which is released when the treponeme damages cells during an infection. This makes positive non-treponemal tests indicative of an active infection, but a confirmatory test with a treponemal test is required to verify that it is indeed a syphilis infection that is causing elevated cardiolipin levels.

The earliest serology test for syphilis was a non-treponemal test, the Venereal Disease Research Laboratory (VDRL) test, that has been in use for nearly 60 years. The Rapid Plasma Reagin (RPR) test that followed was based on the same cardiolipin antigen, but was an improvement over VDRL in that it did not require a microscope to interpret results.

With the advent of new technologies, specifically monoclonal antibodies and recombinant proteins, the range of available serology tests has increased dramatically in the last 20 years and many treponemal tests are now available. Early treponemal tests had their benefits, primarily with increased sensitivity and lower false positives, but were often technically complicated and difficult to perform. Many of the early antibody-based tests such as the fluorescent treponemal antibody-absorption (FTA-ABS) test, Treponema pallidum hemagglutination assay (TPHA and MHA-TP), and Treponema pallidum particle agglutination assay (TP-PA) used the same antibody and were merely refinements on how it was used. This trend has continued with the advent of ELISA/EIA assays as well as lateral flow tests, in that many use identical recombinant antigens.

For more information on syphilis serology tests, please visit our pages on Treponemal and Non-treponemal tests.

While TPHA, TP-PA, and RPR can be performed relatively quickly, they require a laboratory and are not included in our discussion of rapid tests below.

Rapid tests

Rapid treponema-specific tests have become commercially available in the last few years. There are currently over 30 rapid syphilis tests commercially available. However, relatively few of these are currently FDA approved or CE marked. The vast majority of these tests are of the lateral flow format, although some are card-based latex agglutination assays. Since all available rapid tests use T. pallidum recombinant antigens to detect treponeme specific antibodies, the results closely reflect those generated by specific, confirmatory (treponemal) tests rather than nonspecific, screening (non-treponemal) tests. Most of the tests that are available in lateral flow formats are designed to detect all antibody isotypes (IgG, IgM, and IgA) against syphilis. This improvement over previous assay systems ensures that an infection can readily be detected very soon after exposure as well as in its later stages. Overall, the lateral flow platform provides accurate, qualitative detection of antibodies to T. pallidum.

Most syphilis lateral flow tests take 10 to 30 minutes and do not require a laboratory or other instrumentation. Health care staff can easily interpret the results by simple visual examination. The tests use whole blood, serum, or plasma depending on the manufacturer and sample volumes for most assays range from 10 ul to 50 ul. The small volumes allow for a finger-stick sample in place of a venous blood draw, when appropriate. Some tests confer the additional benefit of not requiring refrigeration and have long shelf lives making them an even better option for low-resource settings.

The main disadvantage of rapid tests for syphilis is that, like all other treponemal tests, they will be reactive with virtually every patient who has ever had syphilis, even individuals who no longer have active infection. In areas of low disease prevalence, it may be possible to use these tests as screening tests to identify patients for presumptive treatment. In high-prevalence environments however, a positive lateral flow test would need a confirmatory test with a non-treponemal assay such as a RPR/VDRL or perhaps by DFM when appropriate.

Links to more information