Clinical Significance

Syphilis is primarily transmitted via sexual contact, but can also be transmitted from mother to fetus. Syphilis is caused by the spirochete T. pallidum, which has never been successfully cultured in artificial media. Syphilis infections are classified into early (infectious) and late (non-infectious) stages. Early syphilis may be further divided into primary, secondary, and early latent syphilis. The signs and symptoms of syphilis are numerous; before the advent of serological testing, precise diagnosis was very difficult. In fact, the disease was often confused with other diseases, particularly in its tertiary stage. If not treated, syphilis can cause serious effects such as damage to the heart, aorta, brain, eyes, and bones. In some cases these effects can be fatal. Therefore, the serological diagnosis of syphilis is very important.

The serological diagnosis of syphilis is classified into 2 groups: nontreponemal tests and treponemal tests. Nontreponemal tests, which include venereal disease research laboratory (VDRL) and rapid plasma reagin (RPR) tests, detect antibodies formed by the host in response to lipid material released from damaged host cells as well as to lipoprotein-like material released from the spirochete. Treponemal tests detect specific treponemal antibodies, and the techniques used include agglutination (T. pallidum hemagglutination [TPHA], T. pallidum particle agglutination [TPPA]), immunoassay (enzyme immunoassay [EIA] or chemiluminescent immunoassay [CLIA]), immunofluorescence (fluorescent treponemal antibody absorption [FTA‑ABS]), and immunoblotting. Nontreponemal tests have poor sensitivity and specificity, and recombinant antigen-based treponemal tests have higher sensitivity and specificity than native antigen-based treponemal tests.