Q4:
C. Dilute the patient’s blood 1:100 and repeat the test.

Yes. The test is showing a “prozone” reaction. This is phenomenon that occurs only with agglutination assays. Both the VDRL and the RPR tests for syphilis are agglutination assays. They involve mixing the patient serum with beads that are coated with cardiolipin. If the patient has multivalent antibodies against cardiolipin, the beads agglutinate to indicate a positive test. In this case, the negative test was caused by an excess of antibody, not by a lack of antibody. The diagram below shows how this can happen. Briefly, in the presence of an overabundance of antibody, no crosslinking of the beads occurs, and the test will appear to be negative. The solution to this problem is to dilute the serum. Then, cross-linking and agglutination will occur.