Question
Rapid plasma reagin 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 substance (“reagin”) produced by the host in response to lipid material released from damaged host cells and possibly from the treponemes themselves, which reacts with a cardiolipin-lecithin-cholesterol antigen.
Principle/procedure: patient serum (unheated, unlike VDRL which requires heat-inactivation) is mixed with the cardiolipin antigen, to which fine carbon particles are attached; if reagin antibody is present, flocculation occurs, and the carbon particles clump together, producing macroscopically visible black clumps against the white card background — allowing the test to be read with the naked eye, without a microscope (an advantage over VDRL, which requires microscopic reading).
Use: a rapid, inexpensive screening test, suitable for field/bedside use; used to screen for syphilis and to monitor response to treatment (titres decline with successful therapy, similar to VDRL).
Limitations: like all non-treponemal tests, RPR can give biological false positives — in conditions such as malaria, leprosy, systemic lupus erythematosus, pregnancy, and some other infections/autoimmune conditions — and can be falsely negative in very early primary syphilis (before reagin develops) or due to the prozone phenomenon (excess antibody blocking visible flocculation at low serum dilutions). A reactive RPR must therefore 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.