The RPR-carbon antigen is a non-treponemal preparation specially designed for the detection and semiquantification by macroscopic coagglutination in slide or microplate of plasma reagins, a group of antibodies directed against tissue components produced by patients infected with T. pallidum. The rapid determination of plasma reagins is made by assaying the antigen - a complex association of complex lipids and carbon - against problem samples. The presence or absence of visible agglutination is indicative of the presence or absence of luetic reagins in the tested samples.