The venereal disease research laboratory (VDRL) test is a nonspecific microflocculation test for the screening of syphilis. It uses antigen-containing cardiolipin, lecithin, and cholesterol and measures both IgG and IgM antibodies to lipoidal and lipoproteins released as a result of damage to host cells and also to cardiolipin released from treponemes. VDRL test is easy to perform and inexpensive, so it is commonly used in the screening of the population for syphilis. Without some other evidence for the diagnosis of syphilis, a reactive nontreponemal test does not confirm Treponema pallidum infection.
- Serum (plasma can not be used)
- Cerebrospinal fluid (CSF)
Acceptable CSF and serum specimens should not contain particulate matter that would interfere with reading test results. Serum samples that are excessively hemolyzed, contaminated, and turbid are not suitable for testing.
Principle of VDRL Test
Non-treponemal antigen (cardiolipin-cholesterol-lecithin) is used to detect the presence of “reagin antibodies” (IgM and IgG antibodies to lipoidal material released from damaged host cells as well as to lipoprotein-like material, and possibly cardiolipin released from the treponemes) in the patient’s serum.
When the heat-inactivated serum (to destroy complement) of a patient reacts with freshly prepared non-treponemal antigen, a flocculation reaction (Ag-Ab complex are suspended) occurs. The flocculation can be observed by using a microscope with a 10x objective and 10x eyepiece.
Reactive VDRL test serum can be quantitated to obtain the titre of “reagin antibodies” by using the serial double dilution method.
Result and Interpretation of VDRL test
- Reactive: medium or large clumps
- Weakly reactive: small clumps
- Nonreactive: no clumping or very slight roughness
- Report titers in terms of the highest dilution that produces a reactive (not weakly reactive) result
VDRL test is positive in most cases of primary syphilis and is almost always positive in secondary syphilis. The titer of reagin antibodies decreases with effective treatment, so the VDRL test can be used to determine the treatment response of syphilis.
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Limitation of the Test
A. False positive VDRL test result
- Reagin antibodies may be produced in response to nontreponemal diseases of an acute and chronic nature in which tissue damage occurs such as:
- Hepatitis B
- Infectious Mononucleosis
- Various autoimmune diseases
- VDRL may be reactive in persons from areas where yaws are endemic. As a rule, residual titers from these infections will be <1:8.
- Nontreponemal test titers of persons treated in latent or late stages of syphilis or who have become reinfected do not decrease as rapidly as do those from persons in the early stages of their first infection. In fact, these persons may remain “serofast,” retaining a low -level reactive titer for life.
B. False negative VDRL test
It can be seen because of the prozone phenomenon (no flocculation due to antibody excess). In that case test serum has to be diluted further to obtain a zone of equivalence (where maximum flocculation of Ag-Ab occurs).
Further Reading and References