False Negative Screening Tests for Syphilis in Pregnant Women

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Abstract
A recent increase in the prevalence of syphilis and congenital syphilis has been noted in the United States, reaching epidemic proportions in some areas.1 Controlling this epidemic depends on the identification and treatment of infected women. The true disease state may be misidentified in some clinical situations. False negative results on routine nontreponemal testing may occur in pregnancy during the active disease as a result of the prozone phenomenon. This phenomenon occurs when there is an excess of antibody in the serum samples being examined such that the formation of the antibody—antigen lattice network required for flocculation is prevented.2 3 4 The incidence of the phenomenon is about 2 percent5 in serum samples from patients with high antibody titers, such as are found in cases of primary or secondary syphilis. Dilution of the serum reestablishes the proper concentrations of antibody and antigen to allow the lattice formation required for a test to be positive. We report two cases of pregnant women with syphilis in whom the initial diagnosis was missed as a result of the prozone phenomenon.

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