Unexpected non-reactive nontreponemal serology in secondary syphilis: The prozone phenomenon and clinician–laboratory communication

We describe a case of a 22-year-old woman living with human immunodeficiency virus (HIV) who presented with classical features of secondary syphilis. Initial Rapid Plasma Reagin (RPR) testing was unexpectedly non-reactive; however, repeat RPR testing with serum dilution yielded a reactive result, illustrating the prozone phenomenon. Contribution: The prozone phenomenon is uncommon. We aim to encourage clinician-laboratory communication when there is a high pretest probability of syphilis, with serological discordance, to facilitate prompt diagnosis and treatment.