The Venereal Disease Research Laboratory test (VDRL) is a blood test for syphilis and was developed by the former Venereal Disease Research Laboratory, now the Treponemal Pathogenesis and Immunology Branch, of the United States Public Health Service. The VDRL test is used to screen for syphilis (it has high sensitivity), whereas other, more specific tests are used to diagnose the disease.
The VDRL is a nontreponemal serological screening for syphilis that is also used to assess response to therapy, to detect central nervous system involvement, and as an aid in the diagnosis of congenital syphilis. The basis of the test is that an antibody produced by a patient with syphilis reacts with an extract of ox heart (diphosphatidyl glycerol). It therefore detects anti-cardiolipin antibodies (IgG, IgM or IgA), visualized through foaming of the test tube fluid, or "flocculation".
The RPR (Rapid Plasma Reagin) test uses the same antigen as the VDRL, but in that test it has been bound to several other molecules including a carbon particle to allow visualization of the flocculation reaction without the need of a microscope.
Many other medical conditions can produce false positive results, including some viruses (mononucleosis, hepatitis), drugs, pregnancy, rheumatic fever, rheumatoid arthritis, lupus, and leprosy.
The syphilis anti-cardiolipin antibodies are beta-2 glycoprotein independent, whereas those that occur in the antiphospholipid antibody syndrome (associated to lupus for example) are beta-2 glycoprotein dependent, and this can be used to tell them apart in an ELISA assay. This test is very useful as the trend of titres are correlated to disease activity (i.e. falling titres indicate successful treatment). It has a very good sensitivity for syphilis, except in late tertiary form.