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What tests are used to diagnose syphilis?
There are two types of tests used to diagnose syphilis: treponemal tests (that identify antibodies to the causative organism) and non-treponemal tests (that identify the body's response to the infection but not to the organism itself).
The blood tests used to screen for syphilis are called the Venereal Disease Research Laboratory (VDRL) and Rapid Plasminogen Reagent (RPR) tests. These tests detect the body's response to the infection, but not to the actual Treponema organism. While these are good screening tests, they are not specific enough to establish the diagnosis. These tests can give false-positive results, so a positive screening VDRL or RPR test must be followed by a treponemal test to detect the organism.
A number of different tests are available that detect antibodies to Treponema pallidum bacteria. Examples include the microhemagglutination assay for T. pallidum (MHA-TP) and the fluorescent treponemal antibody absorbed test (FTA-ABS). These tests can confirm the diagnosis of syphilis if a nontreponemal test such as VDRL or RPR is positive.