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Sex Transm Dis. 2016 Dec;43(12):737-740.

Use of Treponemal Screening Assay Strength of Signal to Avoid Unnecessary Confirmatory Testing.

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From the Department of Pathology, University of Texas Medical Branch, Galveston, TX.



Our reverse syphilis testing algorithm consists of a treponemal IgG multiplex flow immunoassay (MFI) followed by both rapid plasma reagin titer and the Treponema pallidum particle agglutination (TPPA) test on specimens with a reactive MFI result. We report here the impact of a modified reverse algorithm, in which the strength of signal of the MFI is used to avoid unnecessary TPPA testing.


The Bioplex syphilis IgG MFI was used as the syphilis screening assay, and specimens with equivocal (antibody index 0.9 or 1.0), or reactive (antibody index ≥ 1.1) results were further tested by rapid plasma reagin titer and TPPA test. We performed a retrospective, descriptive analysis of all specimens received for syphilis screening between January and May of 2014. A cost analysis was performed, taking into account labor and reagent expenses.


In our diverse patient population consisting of high-risk incarcerated persons, low-risk obstetrical/gynecological patients and high-risk miscellaneous clinic and inpatients, 430 (65%) of 665 MFI-positive specimens had antibody indices of 8 or greater. Greater than 99% of these specimens were reactive by the TPPA test. Avoiding TPPA testing of specimens with an MFI antibody index ≥8 would save over US $4800 annually in laboratory costs.


The TPPA testing is unnecessary on specimens with MFI antibody indices ≥8. This would substantially reduce the TPPA testing volume and also reduce laboratory expenses.

[Indexed for MEDLINE]

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