Sex differences in the significance of isolated reactive treponemal chemiluminescence immunoassay results

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The significance of sera with isolated reactive treponemal chemiluminescence immunoassay (IRTCIA) results is unclear. Women have this phenotype more commonly than men. Most cohorts examining this phenotype have included predominantly men and have demonstrated evidence of past or subsequently confirmed syphilis infection in a significant proportion of cases. We hypothesised that a proportion of sera with IRTCIA results would be positive on immunoblot testing and that sera from women with IRTCIA would have different results in immunoblot testing than men.


IRTCIA sera from a tertiary referral serology laboratory serving multiple clinical sites were analysed with a syphilis line immunoblot assay (LIA) and analysed by sex. Logistic regression was undertaken to assess factors associated with LIA status. Medical record review and descriptive analysis of a separate cohort of women with the IRTCIA phenotype from a single campus was also undertaken.


Overall, 19/63 (30.1%) subjects with the IRTCIA phenotype were positive in the LIA, including 13 men and 6 women. Women were significantly less likely to have definitive results (positive or negative) than men (p=0.015). Pregnant women were less likely than non-pregnant women to have a negative LIA result (OR 0.57; p=0.03). Record review of 22 different women with IRTCIA reactivity showed that 2/22 (9.1%) had HIV and previous syphilis infection, 15/22 (68.2%) were pregnant and 3 (13.6%) had autoimmune disease.


A significant proportion of sera with IRTCIA results on serological tests are reactive on LIA testing and some may not be false positive results. The interpretation of IRTCIA results should be undertaken in conjunction with an assessment of factors such as sex, pregnancy, a history of syphilis and other STIs and syphilis risk.

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