Syphilis is a disease, usually sexually transmitted, caused by the spirochete T. pallidum. Infection is systemic from the outset and the disease is characterized by periods of latency, often in excess of twenty years. These features, together with the fact that T. pallidum cannot be readily identified by culture methods, mean that serological techniques play a major role in the diagnosis of Syphilis and treatment follow-up. In response to T. pallidum, two types of human antibody responses normally result: non-specific (anti-Cardiolipin and various lipoidal molecules) and Syphilis specific (anti-Treponemal antigens).
The procedures most commonly used to screen for syphilis in clinical laboratories are based upon their reaction with non-treponemal lipoidal antigens (the reagin tests). Reagin tests, such as the RPR or VDRL, can be used to test serial dilutions of the serum specimen. The end point values from sequentially obtained serum samples decline following successful treatment until after a period of several months the patient will usually become reagin test non-reactive.
Clinical (serum) specimens reactive in reagin tests are typically confirmed using treponemal tests such as the Micro-haemagglutination-T. pallidum (MHA-TP TPHA), T. pallidum Particle Agglutination (TPPA) or the Fluorescent Treponemal Antibody-Absorption (FTA-ABS) test. In contrast to the non-treponemal tests, treponemal test reactivity will persist following treatment in approximately 85% of the cases often for the life of the patient. Any sera giving positive or equivocal results on initial treponemal based assays must be supplemented with a quantitative non-treponemal test (such as RPR or VDRL) to distinguish from active disease and assist in ruling out false positives.
Although MHA-TP TPHA and FTA-Abs are generally considered reliable, their specificity is limited due to the presence of non-specific antigens in the T. pallidum culture preparations. The use of recombinant treponemal antigens results in increased sensitivity and specificity. The antigens detect T. pallidum specific IgG, IgM, and IgA: enabling the test to detect antibodies during all stages of infection.
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