Case report and evaluation of the frequency of the prozone phenomenon in syphilis serology – an infrequent but important laboratory phenomenonJeffrey J. Post A B C E , Candice Khor A B C , Virginia Furner B , Don E. Smith B , L. Ross Whybin D and Peter W. Robertson D
A Department of Infectious Diseases, Prince of Wales Hospital, Randwick, NSW 2031, Australia.
B Albion Street Centre, Prince of Wales Hospital, Surry Hills, NSW 2010, Australia.
C Prince of Wales Clinical School, University of New South Wales, Randwick, NSW, Australia.
D Department of Microbiology, Serology Laboratory, South Eastern Area Laboratory Services, Prince of Wales Hospital, Randwick, NSW, Australia.
E Corresponding author. Email: Jeffrey.Post@sesiahs.health.nsw.gov.au
Sexual Health 9(5) 488-490 https://doi.org/10.1071/SH11156
Submitted: 9 November 2011 Accepted: 1 May 2012 Published: 10 September 2012
Background: Treponema pallidum specific serology generally remains reactive for life. Therefore, the diagnosis of syphilis reinfection relies on clinical assessment and nontreponemal (reagin) serologic testing. The prozone phenomenon can lead to a falsely nonreactive rapid plasma reagin (RPR) assay result. Methods: We report a case of secondary syphilis in a HIV infected patient with a previous history of syphilis infection, where a falsely nonreactive RPR assay was associated with a delayed diagnosis of reinfection and infectious syphilis. The prozone phenomenon was detected in several of the patient’s serum samples collected around this time. We subsequently undertook a prospective evaluation for the prozone phenomenon in 3222 consecutive sera, which were assayed using the RPR assay for clinical purposes over a 10-month period. Results: The overall rate of the prozone phenomenon was 2 out of 3222 samples (0.06%; 95% confidence interval (CI): 0.02–0.22%) and the rate per reactive sample was 2 out of 397 (0.5%; 95% CI: 0.14–1.81%). Conclusion: Clinicians should request RPR testing at dilutions of sera when syphilis is suspected clinically and the RPR assay is nonreactive.
Additional keywords: false negative reactions, HIV, serology, testing.
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