Venereal disease research laboratory (VDRL) test is the serological test used for the identification of Treponema pallidum, causative agent of syphilis infection in the clinical microbiology laboratory. Two types of serological test are available for the detection of T. pallidum antigens in the blood specimen of syphilis patients; and these are:
- Treponemal test.
- Non-treponemal test.
The treponemal test for serological diagnosis of syphilis infection uses a specific antigen to detect antibodies produced against T. pallidum in the serum of syphilis patients. Following the invasion of the body by T. pallidum, the immune system of the human host produces specific antibodies to counter the antigen. This serves as the basis for the treponemal test which uses a specific antigen to target specific antibodies produced in response to T. pallidum invasion of the body. However, this test cannot different between past infection and an active syphilis infection. The Treponema pallidum haemagglutination test (TPHA) and the fluorescent treponemal antibody absorption test (FTA-Abs) are the two available treponemal test for the serological detection of T. pallidum in the clinical microbiology laboratory.
Non-treponemal test include the VDRL and the rapid plasma regain (RPR) tests. Both VDRL and RPR test detect specific antibody-like substances (known as reagin) in the serum of syphilis patients. Reagin is an anti-lipioddal antibody made up of IgM and IgA. Cardiolipin-lecithin coated cholesterol particle (containing micro-particulate carbon/charcoal), a non-treponemal antigen is the antigen used for the VDRL and RPR tests. VDRL and RPR can be used to evaluate both active and past syphilis infections. However, VDRL and RPR tests are not always specific for the serological detection of T. pallidum because the reagin antibody can be present in other serum of people has other related diseases. Only the rapid plasma regain (RPR) test (which is the mostly used serological assay for syphilis diagnosis) which is more sensitive than the VDRL test, easier and faster to perform shall be expanded in this textbook.
Syphilis is a sexually transmitted disease (STD) that is caused by a spirochaete called Treponema pallidum. Natural infection with T. pallidum is only limited to the human host, and human infection is usually transmitted by sexual contact. Congenital syphilis in which the unborn fetus/infant of a pregnant mother is infected by T. pallidum can also occur. It is acquired in utero from the infected mother. Here, a pregnant mother that is syphilitic transmits T. pallidum to her unborn fetus via her placenta beginning in the 10th – 20th weeks of gestation. This can lead to miscarriages, stillborn, and even death.