The VDRL and TPHA tests are used to diagnose syphilis, a sexually transmitted infection caused by the bacterium Treponema pallidum. The VDRL test is a screening test for syphilis that measures substances called antibodies, which your body may produce if you have come in contact with the bacterium. It is a qualitative test for screening of syphilis, and currently all nontreponemal tests are flocculation tests. A positive VDRL test result means you may have syphilis, but the tests ability to detect syphilis depends on the stage of the disease. The tests sensitivity to detect syphilis nears 100% during the middle stages; it is less sensitive during the earlier and later stages. False-positive VDRL test results can occur due to certain infections, connective tissue disorders, or pregnancy.

The TPHA test is a confirmatory test for syphilis that detects human T. pallidum antibodies by a hemagglutination method. It is useful in cases where the VDRL test result is weakly positive or borderline. The results of a study suggest that TPHA positivity is high in sera with VDRL titre less than 1:8. Once positive, always positive holds true for the TPHA test, which means that a positive TPHA test does not necessarily translate to active infection every time. It is best to consult a board-certified dermatologist to correctly diagnose and treat the condition.