Interpretation

Syphilis screening at Mayo Clinic is performed by using the reverse algorithm, which first tests sera for Treponema pallidum specific IgG antibodies using an automated immunoassay. Antibodies to syphilis can remain elevated despite appropriate antimicrobial treatment and a reactive result does not distinguish between recent or past infection. To further evaluate disease and treatment status, samples that are reactive by the syphilis screening test are reflexed to the rapid plasma reagin (RPR) assay, which detects antibodies to cardiolipin, a lipoidal antigen released from host cells damaged by T pallidum. Unlike treponemal-specific antibodies, RPR titers decrease and usually become undetectable following appropriate treatment and can be used to monitor response to therapy.

In some patients, the results of the treponemal screening test and RPR may be discordant (eg, syphilis IgG positive and RPR negative). To discriminate between a falsely reactive screening result and past syphilis, a second treponemal-specific antibody test is recommended using a method that is different from the initial screening test (eg, T pallidum particle agglutination: TP-PA).

In the setting of a positive syphilis IgG screening result and a negative RPR, a positive TP-PA result is consistent with one of the following:

1. Past, successfully treated syphilis

2. Early syphilis with undetectable RPR titers

3. Late/latent syphilis in patients who do not have a history of treatment for syphilis

Further historical evaluation is necessary to distinguish between these scenarios (Table).

In the setting of a positive syphilis IgG screening result and a negative RPR, a negative TP-PA result is most consistent with a falsely reactive syphilis IgG screen (Table). If syphilis remains clinically suspected, a second specimen should be submitted, order SYPH1 / Syphilis IgG with Reflex, Enzyme Immunoassay, Serum.

Table. Interpretation and follow-up of reverse screening results

CIA, chemiluminescence immunoassay; EIA, enzyme immunoassay; N/A, not applicable; RPR, rapid plasma reagin; TP-PA, Treponema pallidum particle agglutination.