Which of the following is not true of the VDRL test?

• A. Inactivated Treponema pallidum serves as the antigen.
• B. False-positive tests are more frequent than with the FTA-ABS test.
• C. The antibody titer will decline if the patient is adequately treated.
• D. The test is usually positive in secondary syphilis.

Answer: (A)

The key idea is that VDRL is a non-treponemal serologic test that uses an artificial antigen, not whole Treponema pallidum. Specifically, the antigen is a cardiolipin-lecithin-cholesterol complex, and it detects reagin antibodies directed against this lipid complex, not antibodies targeting the bacteria itself. Because Treponema pallidum is not used as the antigen, the statement that inactivated T. pallidum serves as the antigen is not true. This design explains why VDRL can yield false positives in other conditions that generate anti-phospholipid antibodies and why its false-positive rate is higher than that of the treponemal-specific FTA-ABS test. It also accounts for the pattern that the antibody titer tends to fall with adequate treatment, and why the test is usually positive in secondary syphilis when antibody levels are high.