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.
The (VDRL) is a nontreponemal serological screening for syphilis that is also used to assess response to therapy, to detect of CNS 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). 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, where as 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.
There are a number of treponemal-specific tests such as the Fluorescent treponemal antibody-absorption (FTA-ABS) test, Treponema pallidum hemagglutination assays (TPHA and MHA-TP), Treponema pallidum particle agglutination assay (TP-PA), and the Toluidine red unheated serum test (TRUST) which may be used to confirm a positive VDRL result. These tests are more specific for syphilis than non-treponemal tests and, in the presence of a positive test, more likely indicate active infection. Unfortunately other treponemal infections such as yaws, bejel and pinta and possibly nonpathogenic commensal treponemes can result in a positive.
However it must be noted that all treponemal specific tests will remain positive for life once a person has been infected with syphilis, even if syphilis has been adequately treated. Therefore, these types of tests cannot be used to monitor the treatment of syphilis.