RPR

RPR commonly denotes Rapid Plasma Reagin, a serologic test used to screen for syphilis. It is a non-treponemal test that detects antibodies produced in response to tissue damage caused by infection with Treponema pallidum. The assay uses a cardiolipin-containing antigen and measures a visible flocculation reaction, typically reported as reactive or nonreactive, with quantitative titers obtained by serial dilution.

In clinical use, RPR serves as a first-line screening test in many settings, including routine prenatal care and sexually transmitted infection programs. It is also used to monitor response to therapy, as decreasing titers over time generally indicate effective treatment, while rising titers may suggest treatment failure or reinfection. Because it is non-specific, a positive RPR is usually confirmed with treponemal-specific tests such as FTA-ABS or TP-PA.

Limitations of the RPR include false positives arising from autoimmune diseases, pregnancy, malaria, other infections, and certain vaccines, as well as false negatives in very early primary syphilis or late-stage disease. The prozone phenomenon—very high antibody concentrations causing a false negative result—can occur and is addressed by testing diluted samples. Interpreting RPR results also requires consideration of the clinical context and patient history.

RPR is often used in conjunction with treponemal tests as part of a two-step testing strategy or as part of reflex testing programs. Treponemal tests tend to remain positive for life, while non-treponemal titers can reflect current disease activity and treatment response.