Question
RPR test
Answer
The Rapid Plasma Reagin (RPR) test is a non-treponemal serological screening test for syphilis, closely related to the VDRL test, detecting antibody-like “reagin” produced by the host against a cardiolipin-lecithin-cholesterol antigen.
Principle/procedure: patient serum (unheated, unlike VDRL) is mixed with cardiolipin antigen to which fine carbon particles are attached; flocculation in the presence of reagin antibody clumps the carbon particles, producing macroscopically visible black clumps against the white card — readable with the naked eye, without a microscope.
Use: a rapid, inexpensive screening test suitable for field/bedside use; also used to monitor response to treatment (titres decline with successful therapy).
Limitations: can give biological false positives (malaria, leprosy, SLE, pregnancy, other infections/autoimmune conditions), and can be falsely negative in very early primary syphilis or due to the prozone phenomenon. A reactive RPR must be confirmed with a specific treponemal test (TPHA, FTA-ABS, or treponemal EIA), which remains positive for life and is not useful for monitoring treatment response.