In contrast to the aggregates formed when particulate antigens bind to specific antibody, the interaction of soluble antigen with antibody may result in the formation of a precipitate, a concentration of fine particles, usually visible only because the precipitated product is forced to remain in a defined space within a matrix. Variations of precipitation and flocculation are widely used for serologic studies.
In flocculation tests the precipitin end product forms macroscopically or microscopically visible clumps. The Venereal Disease Research Laboratory test, known as the VDRL, is the most widely used flocculation test. Patients infected with pathogenic treponemes, most commonly T. pallidum, the agent of syphilis, form an antibody-like protein called reagin that binds to the test antigen, cardiolipin-lecithin–coated cholesterol particles, causing the particles to flocculate. Reagin is not a specific anti body directed against T. pallidum antigens, therefore the test is highly sensitive but not highly specific; however, it is a good screening test, detecting more than 99% of cases of secondary syphilis.
The VDRL is the single most useful test available for testing cerebrospinal fluid in cases of suspected neuro syphilis, although it may be falsely positive in the absence of disease. Performance of the VDRL test requires scrupulously clean glassware and attention to detail, including numerous daily quality control checks. In addition, the reagents must be prepared fresh immediately before the test is performed, and patients’ sera must be inactivated (complement inactivation) by heating for 30 minutes at 56°C before testing. Because of this complexity, the VDRL has been replaced in many laboratories by a qualitatively comparable test, the rapid plasma reagin (RPR) test.
The RPR test is commercially available as a complete system containing positive and negative controls, the reaction card, and the prepared antigen suspension. The antigen, cardiolipin-lecithin–coated cholesterol with choline chloride, also contains charcoal particles to allow for macroscopically visible flocculation. Sera can be tested without heating, and the reaction takes place on the surface of a specially treated cardboard card, which is then discarded (Figure 1). The RPR test is not recommended for testing of cerebrospinal fluid. All procedures are standardized and clearly described in product inserts, and these procedures should be strictly followed. Overall, the RPR appears to be a more specific screening test for syphilis than the VDRL, and it is not as technically complex. Several modifications have been made, such as the use of dyes to enhance visualization of results and the use of automated techniques.
Fig1. MACRO-VUE RPR card test. R, Reactive (positive) test indicated by the diffuse degree of clumping. NR, non-reactive (negative test), indicated by a smooth suspension or non-diffuse slight roughness as demonstrated here as a peripheral roughness in well 1 or somewhat centric roughness in well 2. (Courtesy Becton Dickinson Diagnostic Systems, Sparks, Md.)
Conditions and infections other than syphilis can cause a patient’s serum to yield a positive result in the VDRL or RPR test; these are referred to as biologic false positive tests. Autoimmune diseases, such as systemic lupus erythematosus and rheumatic fever, in addition to infectious mononucleosis, hepatitis, pregnancy, and old age have been known to cause false-positive reactions. The results of screening tests should always be considered presumptive until confirmed with a specific treponemal test.
