Background Recent recommendations propose that serum samples reactive by both syphilis enzyme immunoassay (EIA) or chemoluminescent immonoassay (CLIA) and RPR may not need treponemal confirmatory testing. There is uncertainty regarding the confirmation rate of EIA/CLIA reactive and low titer RPR samples.
Methods Reactive samples for EIA/CLIA and low titer RPR from five Quebec diagnostic laboratories between December 14th 2011 and December 3rd 2012 were prospectively tested with TPPA and, if negative or inconclusive, with a line immunoassay (LIA). Syphilis infection confirmation was defined by a reactive TPPA or LIA.
Results Samples reactive for EIA/CLIA and RPR with titers ranging from 1:1 to 1:8 were submitted for confirmatory testing (N = 345). Of these, 335 (97.1%) were confirmed and 2.9% (95% CI 1.4–5.3%) were misclassified as syphilis cases. When stratifying by RPR titer, unconfirmed samples (misclassified cases) were found only in samples with RPR titer of 1:1 and 1:2. Samples with titers above 1:2 were classified as true syphilis cases. Proportion of confirmed cases increased with RPR titer (p = 0.01).
Conclusions In our setting, only patients with serum RPR titers ranging from 1:1 to 1:2 would have been misclassified as syphilis cases had a confirmatory test not been conducted. A safe and cost-effective approach, for EIA/CLIA reactive and RPR reactive samples management in a reverse sequence algorithm may be to submit only samples with low RPR titers (≤ 1:2) instead of all EIA/CLIA and RPR reactive samples for confirmation.
- Reverse sequence algorithm