52 CLINICAL FOCUS
52 CLINICAL FOCUS
3 NOVEMBER 2023 ausdoc . com . au
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Source : AusDoc website ; 27 Sep to 23 Oct .
An unusual laboratory phenomenon accounts for a false negative result in a suspicious case for syphilis .
Dr Madhara Weerasinghe ( top ) is a GP and an advanced trainee in sexual health medicine at the Northern Sydney Sexual Health Service , Sydney , NSW .
Dr Stephen Davies ( bottom ) is a senior staff specialist sexual health physician at the Northern Sydney Sexual Health Service , Sydney , NSW .
A
60-year-old heterosexual male presents to his GP with a threeweek history of fatigue , arthralgia and night sweats . Blood tests reveal liver function derangement with AST 291U / L ( normal < 40 ), ALT 736U / L ( normal < 35 ), GGT 206U / L ( normal < 50 ), ALP 214U / L ( normal 30-110 ). An autoimmune illness is considered , prompting rheumatology assessment which is inconclusive .
One month later , the patient experiences bilateral reduction in visual acuity and is referred to an ophthalmologist . Ocular examination demonstrates abnormalities in the posterior segment ( choroid ) with visual acuity of 6 / 24 in the right and 6 / 12 in the left .
The ophthalmologist conducts additional blood tests which show reactive syphilis chemiluminescent microparticle immunoassay ( CMIA ) and Treponema pallidum particle agglutination ( TPPA ) but non-reactive rapid plasma reagin test ( RPR ).
The patient is referred to the local sexual health clinic , and on further history , reports unprotected oral and vaginal sex with an anonymous casual female partner one month prior to the onset of his symptoms . He denies any sexual contact with male partners or any recreational substance use . He reports he has had no other sexual contact in the last five years . He has had no prior investigations for sexually transmitted infections ( STIs ) including syphilis .
Assessment
Oral cavity examination reveals a small ulcer (< 1cm ) on the lower gum , which is mildly firm at the base . Neurological and genital examination is unremarkable . There is no rash on the trunk or limbs , and there is full and painless range of movement in all joints .
The community laboratory which conducted the syphilis serology tests is asked to re-do the RPR after diluting the original sample . This returns a revised result of RPR reactive , titre 1:512 , confirming the prozone phenomenon and making the diagnosis of ocular syphilis highly likely .
The ulcer on the lower gum is swabbed and is positive for Treponema pallidum DNA . RPR is requested from the hospital laboratory ( which routinely performs a parallel RPR test on a diluted sample at 1:16 ), and is reported as reactive , titre 1:256 .
The patient screens negative for other STIs , including HIV , hepatitis B , hepatitis C , chlamydia and gonorrhoea .
Discussion
The prozone phenomenon occurs when high concentrations of an antibody in the serum interferes with antigen-antibody complex formation , which forms the basis of a positive flocculation test , such as the RPR . 4 Although
uncommon , this can lead to false negative results with subsequent missed or delayed diagnoses , incorrect staging and onward transmission .
The prozone phenomenon is estimated to occur in 0.2-2 % of syphilis cases and is associated with HIV-coinfection , early stage of infection , neurosyphilis and pregnancy . 5 It
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