Serologic Testing for Syphilis
Published: April 2011
Hot Topic Q&A is an opportunity for viewers to submit questions to the Hot Topic presenter. The opportunity to submit questions for this topic is now closed.
The following questions were submitted by viewers and answered by the presenters, Matt Binnicker, PhD from the Division of Clinical Microbiology at Mayo Clinic in Rochester, Minnesota. Questions are presented as submitted (unedited).
What is the monetary impact on labs that perform reverse screening versus labs that do the traditional screening?
It is still unclear the financial impact of reverse screening compared to testing by the traditional algorithm. This is a complex issue with many factors that need to be considered when determining the true cost. For example, utilization of the reverse screening algorithm typically involves the purchase of expensive automated instrumentation. In addition, treponemal tests using enzyme immunoassay (EIA) or multiplex flow immunoassay (MFI) technology cost more than nontreponemal tests (e.g., RPR). However, automated testing reduces the time required by laboratory personnel to perform testing, so this may significantly reduce cost associated with “tech-time.” Currently, the reverse screening algorithm may be most suitable for high-volume reference labs, while the traditional algorithm may be more cost-effective for low-volume, hospital-based laboratories.
What algorithm for syphilis is used for reproductive donors (oocyte and sperm) and what are the interpretations?
Donor testing (either blood products or reproductive products) should be performed at an accredited donor testing facility. The algorithm used for donor testing varies depending on the institution, volume of samples tested, and prevalence of disease. Mayo Medical Laboratories only performs syphilis testing for diagnostic (patient) purposes, not donor services.
Focusing diagnosis in pregnant women.... IgG tests are specifically what? How should physicians request these?
For routine screening during pregnancy, syphilis serology may be performed by either the traditional or reverse screening methods. When screening is performed by the reverse algorithm, an IgG or total treponemal antibody test should be used. These tests include those based on enzyme immunoassay (EIA), multiplex flow immunoassay (MFI), or chemiluminescence immunoassay (CIA) technology. In addition, tests such as FTA or TP-PA are treponemal tests that detect IgG or total antibodies directed against Treponema pallidum. This is in contrast to the nontreponemal tests (eg, RPR or VDRL), which are designed to detect antibodies to non-specific antigens (eg, cardiolipin).
When a treponemal IgG or total antibody screening test is positive, the same sample should be tested by RPR. If positive, this would suggest active or recently treated syphilis. If the RPR is negative, then a second treponemal test (e.g., TP-PA) should be performed to rule-out a false-positive IgG screening test. If the TP-PA is positive in a patient with unknown history of syphilis or treatment for syphilis, late/latent syphilis should be considered.
Does Mayo Medical Labs offer both FTA and TP-PA assays?
Currently, Mayo Medical Laboratories does not offer FTA or TP-PA. However, we have completed the validation testing for TP-PA, and plan to bring TP-PA in-house by the fourth quarter of 2011. The TP-PA will be automatically performed on samples testing positive by the screening syphilis IgG test but negative by RPR. This approach has been recommended by the Centers for Disease Control when the reverse screening algorithm is followed.
I do not understand the purpose of the contemporary algorithm, it does not make sense; seems like a waste of time and money.
The contemporary algorithm has been adopted by many laboratories to meet the high volumes of testing for syphilis. The RPR test (used for screening in the traditional algorithm) is a highly manual and subjective test, rendering it difficult for high-volume reference labs to use for routine screening. Newer treponemal tests (e.g., enzyme immunoassay or multiplex flow immunoassay) may be automated, which allows for laboratories to test more samples in a shorter period of time. In addition, treponemal tests are often times more specific than RPR. This may yield a reduced number of false-positive results, especially in low-prevalence settings.