Literature review > Issue 9 > Review on Schmidt et al. 

Treponemal tests, while necessary to confirm either nontreponemal test results or clinical impressions, require several hours to perform. Although their use is reasonable for a reference or clinical laboratory, it is not acceptable for clinics where results are needed before the patient leaves the premises. In such settings, a test is needed that gives results within 20 to 30 minutes and which has sensitivity and specificity comparable to the traditional treponemal tests.

In this paper, the author compared a new gelatin particle gel immunoassay (PaGIA) to several established tests for the detection of syphilis antibodies. In this assay, gel particles are coated with the recombinant antigens TpN15, TpN17, and TpN47. Serum sample and coated gel particles are pipetted to the funnel portion of a microtube which contains a gel matrix. After a 5 minute incubation, followed by a 10 minute centrifugation, results are read visually. A positive reaction consists of a red band at or slightly below the top of the matrix; a nonreactive result has a red button at the bottom of the gel matrix. A total of 124 syphilitic serum samples, all but 5 from early syphilis (<1 yr duration), were tested along with 432 fresh samples from persons without a history of syphilis, 20 sera from patients with Lyme Disease, and 38 false positive (reactive serologic test with no history of syphilis) serum samples. The tests that the PaGIA was compared to were the Venereal Disease Research Laboratory (VDRL), syphilis enzyme immunoassay (ICE EIA), and Treponema pallidum particle agglutination (TP-PA). There were 16 serum samples from patients with either primary or early latent syphilis that were nonreactive in the VDRL and/or TP-PA. These 16 samples were also tested in the fluorescent treponemal antibody absorption (FTA-ABS), Captia IgM enzyme immunoassay, and 19S IgM FTA-ABS.

For the total number of syphilis samples tested, the sensitivities of the treponemal based tests were all over 90% (range 91.9% [PaGIA] to 96% [TP-PA]) while the VDRL was 87.8%. However, for the 16 samples (13 primary and 3 early latent) that were nonreactive in the VDRL (15/16) or TP-PA (5/16), the results were more variable. The FTA-ABS and Captia IgM were the most sensitive (93.8%), followed by the 19S IgM FTA-ABS (87.5%), the ICE EIA (56.2) and the PaGIA (50%). Sensitivity for the TP-PA was 62.5% and the VDRL 6.2% for this group. Specificity for the PaGIA was 99.8%.

Although the number of syphilis samples tested was adequate for this type of evaluation, the author unfortunately did not include any samples from patients with late latent syphilis. Since this is frequently the group that is difficult to diagnose, determination of the sensitivity of the PaGIA for this group would have been useful. In addition, comparative specificities for the other tests would also have been useful. Did the PaGIA detect fewer false positives than the other tests? It would also have been useful to know what caused the false positives in the VDRL and FTA-ABS, although that information may not have been available. Inclusion of some sera from persons who were IV drug users or who had autoimmune disease would have added to the characterization of the specificity as well. There also was no indication of how many samples could be tested at one time.

Overall, the author's data indicate that the PaGIA test would be useful in situations where the number of tests performed on a daily basis is not excessive and where quick results are advantageous since the total time for the test is about 20 minutes. The biggest disadvantage to the PaGIA test is the need for a specialized centrifuge to obtain the results. The sensitivity and specificity appear to be comparable to other treponemal tests but with the added advantage of minimal time required to perform the test.

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