Survey of Syphilis Testing Algorithms Using Treponemal Screening Tests

Background:
Syphilis testing in the US has traditionally consisted of screening with an inexpensive non-treponemal test, with all positive results confirmed by a more specific, but more expensive, treponemal test. The introduction of automated treponemal enzyme and chemiluminescence immunoassays has reversed the testing sequence – a screening treponemal test then a confirmatory non-treponemal test.

Objective:
Describe existing screening algorithms beginning with a treponemal test and the approach to individuals with a positive treponemal and a negative non-treponemal test.

Method:
Data collection from four NYC laboratories that routinely conduct syphilis screening using EIAs, and were able to provide testing algorithms, and number of syphilis tests performed over a defined period.

Result:
Each laboratory used a slightly different testing algorithm. Of 116,822 specimens, 6,587(6%) were initially positive by EIA. Positive specimens were tested with RPR, 3,664/6,548 (56%) were positive. Further testing with FTA-ABS or TPPA tests on EIA positive specimens, but negative on RPR found 2,079/2,512 (83%) confirmed treponemal positive. One laboratory performed TPPA testing on specimens positive by EIA and RPR tests, 78/80 (98%) confirmed positive.

Conclusion:
Laboratories used a variety of algorithms, and had different approaches to specimens that test positive with treponemal tests and negative with non-treponemal tests (approximately 3% of specimens).

Implications:
In areas where the use of the EIA is common, STD programs may be asked for guidance on test interpretation, and will need to decide whether to follow up on positive treponemal, negative non-treponemal specimens. Asymptomatic persons with positive treponemal and negative non-treponemal tests are unlikely to be infectious and should be a low priority for health department follow-up. If they have never been treated for syphilis, they have a small, unknown, risk of developing tertiary syphilis, so treatment decisions are based on incomplete information. Further analyses of these approaches are needed.