Prozone phenomenon in secondary syphilis with HIV co-infection

Prozone phenomenon is defined as a false-negative response resulting from higher antibody titer which interferes with formation of antigen-antibody lattice, necessary to visualize a positive flocculation test. The prozone effect can be observed in syphilis testing with cases of very high antibody titers, such as secondary syphilis, or with human immunodeficiency virus (HIV) co-infection. We report two cases of prozone phenomenon in secondary syphilis with HIV co-infection who initially tested nonreactive for rapid plasma reagin test but tested positive with further higher dilution.