Syphilis screening traditionally uses a non-treponemal test such as the RPR (rapid plasma reagin) or VDRL (venereal disease research laboratory) test. If the screening test is positive, a treponemal test, such as TP-PA (Treponema pallidum particle agglutination test), is performed. These are time intensive tests for laboratories to perform as they are done by hand.
Many laboratories have transitioned to using a “Reverse Sequence” for syphilis screening using immunoassays because they are automated. The first test is a treponemal test, either an EIA (enzyme immunoassay) or a CIA (chemiluminescent immunoassay). If this assay is positive, it is followed by a non-treponemal test.
This CDC webpage has more information on Reverse Sequence Syphilis Screening including a webinar and slide set with testing algorithms, a Dear Colleague letter and MMWR report on Discordant Results from Reverse Sequence Syphilis Screening in the USA 2006—2010
Frequently Asked Questions about Reverse Sequence Syphilis Screening are also available from the California STD Control Branch