VDRL test: Principle, Procedure, Result interpretation and Application
Principle:
- VDRL stands for Venereal Disease Research Laboratory test. It is used for serological diagnosis of syphilis and it is an example of Slide flocculation test. In this test cardiolipin antigen is used as reagent to detect auto-antibody in serum of patients.
- Cadiolipin antigen is an alcoholic extract of bovine heart muscle to which lecithin and cholesterol are added. Cardiolipin reacts non-specifically to cardiolipin auto-antibodies (IgM and IgG) to form flocculation (floccular agglutination).
- Autoantibody in response to cardiolipin is produced within 2-3 weeks of treponemal infection due to tissue damage. The stimulating antigen for auto-antibody production is mitochondrial membrane. These autoantibodies can react with cardiolipin.
- The autoantibodies are not only produced in case of syphilis but also in other treponemal infection, occasionally during pregnancy, in acute malaria, leprosy, tuberculosis, viral pneumonia, leptospirosis, cancer, infectious mononucleosis, and sometimes after certain vaccination. So this VDRL test is non –specific test. For the test, at first a drop of antigen is placed on a slide and then a drop of serum is added to it. The slide is rotated to mix the content. In case of positive test, flocculation occurs.
Procedure:
- Take patient’s serum, heat is for 30 minutes at 56°C and then allow to cool to room temperature
- Take a measured volume of diluted antigen suspension (a colloidal suspension of tissue cardiolipid or chemically synthesized cardiolipin) and add to a measured volume of serum in a glass slide.
- Rotate the slide for about 4 minutes,
- Examine the slide microscopically for clumping of Ag-Ab complex using 10X objective and eye piece.
Result interpretation:
- Reactive (Positive test): If Ag-Ab clump of large or medium size then it is reported as reactive (Positive VDRL test)
- Weakly reactive: If Ag-Ab complex is small sized, it is weakly reactive
- Non-reactive: If no Ag-Ab clump is formed, it is non-reactive
- All reactive and weakly reactive serum require serial dilution to estimate antibody titer.
Application:
- VDRL test is most widely used simple and rapid test for syphilis.
- It is a non-treponemal test (non-specific test) used to investigate syphilis, other treponematose, Yaws and Pinta.