A CDC analysis of data from five US laboratories that used an alternative method of screening for syphilis found it returned a higher number of false-positive results than expected.
Some labs seeking to reduce time and labor required for syphilis screening have adopted a reverse sequence of screening in which the treponemal enzyme and chemiluminiescence immunoassays (EIA/CIA) is performed first, followed by testing of reactive sera with a nontreponemal test. CDC recommends screening with a nontreponemal test like the rapid plasma reagin (RPR) or Venereal Disease Research Laboratory (VDRL) test, followed by confirmation using one of several treponemal tests.
From 2006 to 2010, CDC researchers looked at data on reverse sequence testing in populations with high and low syphilis prevalence. Three sites served patient populations with low syphilis prevalence (large managed care organizations), and two sites served populations with high prevalence (including people with HIV and men who have sex with men).
A total of 140,176 specimens screened with EIA/CIA were included; of these, 4,834 (3.4 percent) had a reactive test result. Among them, 2,743 (56.7 percent) were RPR-nonreactive, with additional confirmatory treponemal testing showing 866 (31.6 percent) were nonreactive, suggesting the initial EIA/CIA result was false-positive.
According to CDC, “Among discordant sera, the rate of nonreactive confirmatory treponemal tests was 2.9 times higher in a population with low prevalence of syphilis, suggesting that the low-prevalence population had a higher percentage of false-positive results.”
Reverse sequence screening does have one advantage: “It doesn’t miss people who are infected,” said Dr. Karen Hoover, a CDC epidemiologist and study co-author.
“CDC continues to recommend that nontreponemal tests be used to screen for syphilis and that treponemal testing be used to confirm syphilis as the cause of nontreponemal reactivity. … However, if reverse sequence screening is used, CDC recommends that a specimen with reactive EIA/CIA be tested reflexively with a quantitative nontreponemal test (e.g., RPR or VDRL). If test results are discordant, the specimen should be tested reflexively using the [Treponema pallidum particle agglutination] test as a confirmatory treponemal test,” the authors concluded.
The report, “Discordant Results from Reverse Sequence Syphilis Screening — Five Laboratories, Unites States, 2006-2010,” was published in CDC’s Morbidity and Mortality Weekly Report (2011;60(5):133-137).
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