Young man with unexplained hair loss

While syphilis can be diagnosed by direct detection of the treponemal spirochete under dark-field microscopy, it is usually identified by one of 2 quick and inexpensive serologic screening tests: an RPR or a venereal disease research laboratory (VDRL) test. These tests can be positive as early as 7 days after the appearance of the original chancre. Due to the possibility of a false positive result caused by a viral infection, tuberculosis, or connective tissue diseases, confirmatory testing with fluorescent treponemal antibody absorption (FTA-ABS) or a Treponema pallidum hemagglutination assay (TPHA) is necessary.²

Alopecia in a “moth-eaten” pattern is a less common finding of secondary syphilis.

Our patient initially had several false negative RPR tests. His RPR at the time of his visit to our dermatology clinic was also negative. This was due to the prozone phenomenon, which occurs when a high antibody titer interferes with the formation of an antigen-antibody lattice, which is needed for a positive flocculation test. The incidence of this phenomenon ranges from 0.2% to 2%,¹ and it is commonly reported with HIV coinfection and pregnancy. If syphilis is suspected in a patient, a negative RPR should prompt requests for the laboratory to dilute the patient’s serum to ensure that the prozone phenomenon does not result in a false negative.

Because we highly suspected syphilis in our patient, we requested his serum be serially diluted. The final RPR titer was positive (1:128), and a confirmatory FTA-ABS was also positive.