Rapid plasma reagin
The rapid plasma reagin (RPR), RPR titer, or RPR test 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. 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 RPR test is an effective screening test, as it is very good at detecting syphilis in people without symptoms. However the test may suggest that people have syphilis who in reality do not (i.e., it may produce false positives). False positives can be seen in viral infections (Epstein-Barr, hepatitis, varicella, measles), lymphoma, tuberculosis, malaria, endocarditis, connective tissue disease, pregnancy, autoimmune diseases, intravenous drug abuse, or contamination. It can also occur naturally in the elderly. 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. Unfortunately, false positives can still occur in related treponomal infections such as yaws and pinta. Tests based on enzyme-linked immunoassays are also used to confirm the results of simpler screening tests for syphilis.
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, or as replacements for, the rapid plasma reagin test. One of these alternatives is an immunochromatographic strip test. A study published in February 2006 found that this test outperformed the RPR test in values of sensitivity and specificity, and it does not require a laboratory to process the results.
FTA-ABS is the most specific test for syphilis. If this is positive it confirms the diagnosis.
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