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RPR (Rapid Plasma Reagin)

The RPR is the most commonly used non‑treponemal screening test for syphilis. Reagin is not a specific antibody to Treponema pallidum, but is an IgM antibody to tissue lipids. It has been suggested that T. pallidum damages host tissues, splitting off lipids, which combine the proteins of the spirochete. They, then act like haptens and stimulate the production of reagin.

Reagin titers are negative or low early in primary syphilis. As the disease progresses, titers increase and a greater percentage of patients develop a reactive RPR. RPR is positive in 70% of primary stage cases, 98 - 100% of secondary stage cases, but only 1% of latent cases.

The FTA-ABS is reactive earlier in primary syphilis than the RPR and remains reactive in 98% of patients with latent syphilis. RPR titers may decrease after several years, even without therapy. RPR titers drop rapidly after treatment of primary or secondary syphilis, while the FTA-ABS remains reactive, irrespective of treatment.

RPR is not highly specific. Biological false positive reactions (BFP) occur in about 1 in 4,000 persons in the general population and 1 in 2,000 pregnant women. Both acute and chronic biologic false positive reactions occur. Viral and bacterial infections, immunizations, and pregnancy may cause acute BFP. The titers are usually low and decrease with time. Autoimmune diseases, senescence, cirrhosis, metastatic cancer, lymphomas, myeloma, drug abuse, and anti-cardiolipin antibody syndromes cause chronic BFP. Reactivity usually persists more than six months and the titer remains fixed at a low level.

Reference value is nonreactive. All reactive RPR tests will be automatically followed by a confirmatory test (FTA‑ABS). Many states within the United States mandate reporting of all reactive tests.

Specimen requirement is one SST tube of blood.

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