Treponemal Tests and Reverse Sequence Algorithm
Most patients who have reactive treponemal tests will have reactive tests for the remainder of their lives, regardless of adequate treatment or disease activity. However, 15%–25% of patients treated during the primary stage revert to being serologically nonreactive after 2–3 years (570). Treponemal antibody titers do not predict treatment response and therefore should not be used for this purpose.
Clinical laboratories sometimes screen syphilis serologic samples by using automated treponemal immunoassays, typically by EIA or CIA. This reverse sequence algorithm for syphilis testing can identify persons previously treated for syphilis, those with untreated or incompletely treated syphilis, and those with false-positive results that can occur with a low likelihood of infection. Persons with a positive treponemal screening test should have a standard quantitative nontreponemal test with titer performed reflexively by the laboratory to guide patient management decisions. If the nontreponemal test is negative, the laboratory should perform a treponemal test different from the one used for initial testing, preferably TP-PA or treponemal assay based on different antigens than the original test, to adjudicate the results of the initial test.
The improved Solaris Diagnostics laboratory Syphilis Screening approach is provided in Figure 2 which provides the following advantages.
- Treponemal IgG screening tests are highly sensitive and specific.
- Fewer false negatives than non-treponemal assays, presumably due to detection of latent syphilis.
- Increased testing volume while reducing manual labor.
- More effective for diagnosis of secondary, latent, and late syphilis as they are not subject to prozone reactions, which have been reported as a rare occurrence in nontreponemal (RPR) assays. The high antibody titer which interferes with formation of antigen-antibody lattice needed to visualize a positive flocculation or clumping test. Prozone effect is most often associated with secondary syphilis, HIV co-infection and pregnancy.