Large liposome agglutination technique for the serological detection of syphilis

In response to tissue invasion by Treponema pallidum, an antibody complex, called reagin, appears in the serum of the syphilitic patients. Reagin has the ability to combine with cardiolipin, therefore, cardiolipin has traditionally been used as the antigen in different test configurations for the serological detection of syphilis: the VDRL (Venereal Disease Research Laboratory) flocculation test and the RPR (rapid plasma reagin) test. Here we introduce a liposome preparation composed of cardiolipin, lecithin and cholesterol and containing a colored dye which upon mixing with syphilitic serum on a slide or a plastic card results in clearly visible liposome agglutination. We show that the liposomes in the size range of 1–10 μm are essential for proper sensitivity. The liposomes are colored with either a water-soluble dye entrapped in the aqueous space of liposomes or a lipophilic dye embedded in the membrane. By virtue of the liposome size and coloration the agglutination pattern is easily visible within 3–5 min without the use of a mechanical rotator. The approach described here can be extended to the detection of other antibodies and polyvalent antigens.