VDRL (Venereal Disease Research Laboratory Test) is diagnostic blood test used to determine the presence of active Syphilis infection. Thus, it is a screening test. Syphilis is a disease caused by the bacteria Treponema pallidum. The patient’s immune system will produce antibodies in response to the infection. VDRL test involves the measurement of level of these antibodies produced against the syphilis bacteria.
Alternative name for VDRL is Venereal Disease Research Laboratory slide test.
The VDRL test is a blood test. In case of Neurosyphilis which affects the patient’s brain, the test is carried out on spinal fluid or CSF. This test is carried out as a slide flocculation test. It depends on the formation of floccules. The flocculation occurs between antibodies present in the patient’s blood and the antigen present in the test reagent. The type of antigen generally used is the cardiolipin antigen. This antigen is a lipid extract obtained from beef heart. Hence, the VDRL test is not always specific and is also known as non specific or non-treponemal test.
The patient’s blood is initially centrifuged to obtain the serum. The serum is then inactivated by keeping in hot water bath for 30 minutes at 56˚C. This inactivation step helps in inactivating or removing all the non specific inhibitor’s present in the serum. The test is usually carried out qualitatively. But if the qualitative test shows positive result, the sample will be tested quantitatively.
In qualitative test 0.05ml of serum is added to one drop of antigen. This mixture is then rotated for 4 minutes at 180 rotations per minute in a VDRL rotator. Along with this a positive and negative control sample should also be tested to detect the quality of the test antigen. The wells are then observed through a microscope to detect any flocculation reaction.
If the qualitative test gives positive result, the quantitative test is performed to detect the titer of antibody present in the patient’s sample. In quantitative test, the patient’s serum is first serially diluted. From each dilution 0.05 ml of sample is added to separate wells followed by one drop of antigen. The wells are then rotated for four minutes at 180 rotations per minute. These wells are then observed under a microscope and the highest dilution that has flocculation is reported as the reactive titer.
The positive result is usually obtained one to two weeks after the primary lesion has appeared. Only during the late stage of primary syphilis the test gives a 60-80% positive result. The test usually gives a strong positive result during the second phase of syphilis. After the second phase, the reactivity gradually decreases.
If the disease is diagnosed and treated in the primary phase itself, the antibody production gets suppressed and hence gives a non reactive result. On the other hand, treatments in the latent phase does not bring much effect on the antibody titer.
Since there are chances where the VDRL test may sometimes give a false positive result, the disease should be confirmed only by correlating with the clinical symptoms. Similarly, due to the use of non-treponemal antigens for the test, the test may sometimes give false positive result.
VDRL test is performed to detect or diagnose the Treponema pellidum infection. In case of pregnancy, the physician may ask to perform the VDRL test as a routine prenatal care check-up. In some cases, this test is needed for obtaining the marriage license or marriage certificate. Apart from its use to diagnose new infection, the physician may ask to perform the test to monitor the effectiveness of the treatment in patients who are found to be positive for syphilis. The anti cardiolipin antibody titer may vary depending on the severity of the infection and effectiveness of the medication. VDRL test also helps in screening congenital syphilis. In newborns, the test may sometimes give false positive result. This is due to the transfer of antibodies from the infected mother to the child. In such case, the test should be repeated after a month to confirm the result.
A negative result indicates absence of the antibodies specific to syphilis bacteria that shows the absence of infection.
If the VDRL test result is positive, it means the patient suffers from syphilis. Once the positive result is obtained, the physician will ask you to perform TPHA (Treponema pellidum Hemeagglutination Assay) or FTA-ABS test. FTA-ABS test helps in further confirming the infection since it is more accurate.
The effectiveness of VDRL test may vary depending on the various stage of the disease. In some patients the latent infection by Treponema pallidum may give a false negative result. The VDRL test may sometimes give a false positive result even after the disease has been cured. In patients suffering from HIV, Lyme disease, pneumonia, systemic lupus erythematosus etc., the VDRL test mostly gives false positive result. Thus, the VDRL test will help to detect that the person has been infected and not to specifically rule out the current infection.