How do RPR and VDRL tests detect evidence of syphilis?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Scanning electron micrograph of Treponema pallidum, the spirochete bacterium that is the causative agent of syphilis. — syphilis Scanning electron micrograph of *Treponema pallidum*, the spirochete bacterium that is the causative agent of syphilis.

The Rapid Plasma Reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests are classified as nontreponemal serological tests used to detect syphilis.

These tests function by identifying the presence of reagin in a patient's blood serum. Reagin is a type of antibody produced by the body in response to the infection. Detection occurs through a reaction between the reagin and a lipid antigen, typically derived from beef heart. When these components react, they produce a visible clumping effect known as flocculation.1

Because these tests rely on the body producing a sufficient amount of reagin—which generally occurs several weeks after the appearance of a chancre—they are not immediate indicators of infection. Consequently, a positive result from an RPR or VDRL test typically requires confirmation through a more specific treponemal test, such as the Treponema pallidum hemagglutination assay (TPHA).1