16.3.2: The RPR Test for Syphilis

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Syphilis is a sexually transmitted disease caused by the spirochete Treponema pallidum. The RPR (Rapid Plasma Reagin) Card® test is a presumptive serologic screening test for syphilis. The serum of a person with syphilis contains a non-specific anti-lipid antibody (traditionally termed reagin), which is not found in normal serum. The exact nature of the anti-lipid (reagin) antibody is not known but it is thought that a syphilis infection instigates the breakdown of the patient's own tissue cells. Fatty substances which are released then combine with protein from Treponema pallidum to form an antigen which stimulates the body to produce antibodies against both the body's tissue lipids (non-specific or non-treponemal) as well as the T. pallidum protein (specific or treponemal). The RPR Card® test detects the nonspecific antilipid antibody and is referred to as a non-treponemal test for syphilis.

It must be remembered that tests for the presence of these nonspecific antilipid antibodies are meant as a presumptive screening test for syphilis. Similar reagin-like antibodies may also be present as a result of other diseases such as malaria, leprosy, infectious mononucleosis, systemic lupus erythematosus, viral pneumonia, measles, and collagen diseases and may give biologic false-positive results (BFP). Confirming tests should be made for the presence of specific antibodies against the T. pallidum itself. The confirming test for syphilis is the FTA-ABS test discussed below. Any serologic test for syphilis is referred to commonly as an STS (Serological Test for Syphilis).

The known RPR antigen consists of cardiolipin, lecithin, and cholesterol bound to charcoal particles in order to make the reaction visible to the naked eye. If the patient has syphilis, the antilipid antibodies in his or her serum will cross-react with the known RPR lipid antigens giving a visible clumping of the charcoal particles (see Fig. \(\PageIndex{1}\)).

Photograph of a positive RPR test for syphilis showing clumping of carbon particles. — Figure \(\PageIndex{1}\): **Positive RPR Serologic Test for Syphilis**. Clumping of the carbon particles indicates the person's serum contains nonspecific antilipid (reagin) antibodies.. (Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0)

We will do a quantitative RPR Card® test today in lab. Keep in mind that a quantitative test allows one to determine the titer or amount of a certain antibody in the serum. In this test, a constant amount of RPR antigen is added to dilutions of the patient's serum. The most dilute sample of the patient's serum still containing enough antibodies to give a visible antigen-antibody reaction is reported as the titer.

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)