Acute Flaccid Paralysis Accompanying West Nile Meningitis Ahmed ...

Prozone Phenomenon
Ferman, E, DO
St. Michael’s Medical Center, Newark, NJ
Introduction: Nontreponemal tests are useful screening tests for syphilis
infection. RPR is 70-80% sensitive for syphilis and can be falsely negative in
patients with secondary syphilis. This case illustrates prozone phenomenon
masking a syphilis infection.
Case: A 28-year-old homosexual male with no prior medical history presented to
an ambulatory clinic for an annual checkup and had a 3 week history of a grey
rash that began on his thighs, spread to his upper arms, and has been resolving
on its own without any treatment. His last sexual encounter was 8 months ago.
Physical examination showed a macular, nonpruritic rash that was grey in color
and involved the thighs and proximal upper extremities bilaterally as well as a 1
cm x 0.5 cm ulcer on the dorsal shaft of the penis with a granulomatous base.
The skin was otherwise intact without lymphadenopathy and the remainder of the
examination was otherwise unremarkable. Serum chemistry, liver function tests,
and a hepatitis panel were unremarkable. HIV test was negative, and RPR
screen was nonreactive. Given the high index of suspicion for secondary
syphilis, RPR was repeated in higher dilution and found to be positive at 1:512.
The patient was treated with intramuscular Benzathine Penicillin G injected
weekly for 3 weeks. This therapy was tolerated well and his rash resolved over
this period of time.
Discussion: The prozone phenomenon is a rare but important entity to
recognize to avoid missing the diagnosis of secondary syphilis. Prozone
phenomenon is not routinely ruled out in all laboratories. Thus, when serology is
not consistent with the clinical presentation of syphilis, it should be repeated in
higher dilution.