Rapid plasma reagin
The rapid plasma reagin (RPR), RPR titer, RPR test, or Nontreponemal refers to a type of rapid diagnostic test that looks for non-specific antibodies in the blood of the patient that may indicate a syphilis infection. The term reagin means that this test does not look for antibodies against the actual bacterium, Treponema pallidum, but rather for antibodies against substances released by cells when they are damaged by T. pallidum. Traditionally, syphilis serologic testing has been performed using a NTT such as the RPR or VDRL test, with positive results then confirmed using a specific TT such as TPPA or FTA-ABS (Fig. 1a). This algorithm is currently endorsed by the U.S. Centers for Disease Control and Prevention (CDC). In addition to screening for syphilis, a titer can be used to track the progress of the disease over time and its response to therapy. The traditional algorithm using a nontreponemal test (NTT) followed by a treponemal test (TT) remains the standard in many parts of the world.
The RPR test is an effective screening test, as it is very good at detecting syphilis in people without symptoms.  As a result, these two screening tests should always be followed up by a more specific treponemal test. Tests based on monoclonal antibodies and immunofluorescence, including T. pallidum hemagglutination assay (TPHA) and fluorescent treponemal antibody absorption (FTA-ABS) are more specific and more expensive. Nontreponemal tests.NTT measure levels of immunoglobulin G (IgG) and immunoglobulin M (IgM) antibodies produced by the host in response to lipoidal material (mostly cardiolipin) released from damaged host cells. It also generally believed that some cardiolipin is released by the spirochetes as well.
Another test often used to screen for syphilis is the Venereal Disease Research Laboratory (VDRL) slide test. However, the RPR test is generally preferred due to its ease of use.
Other types of tests are currently being evaluated as possible alternatives to standard manual RPR testing. One of these alternatives is a fully automated RPR syphilis analyzer.
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