Determination of a Sample-to-Cutoff Ratio to Predict True-Positivity in Blood Donor Samples Screened for Syphilis by a Chemiluminescent Immunoassay

Abstract

Purpose: The use of Architect Syphilis TP (CMIA) in the blood bank raised the number of syphilis positive samples requiring confirmation. The aim of this study is to determine a sample-to-cutoff (s/co) ratio for CMIA predicting >= 95% of true-positive samples to reduce these samples. Methods: CMIA reactive samples (n=177) were evaluated by Western blot (WB) as the reference standard, as well as by Treponema pallidum hemagglutination (TPHA) and Rapid Plasma Reagin (RPR) tests. The s/co ratio predicting >= 95% of true-positive samples was defined as the threshold leaving >= 95% of WB confirmed samples greater than the particular value. The performances of TPHA and RPR tests were also evaluated with respect to s/co ratios of CMIA positive samples. Results: The s/co ratio 15.17 predicted a true-positive result for >= 95% of samples tested (95% confidence interval: 85.9-99.3) and reduced the number of samples requiring confirmation by 29.9%. Higher s/co ratios were correlated with the increasing number of bands on WB strips (p<0.0001, R=0.906). For the samples with s/co ratios between 3 and 15.17, the agreement of TPHA and WB test results were 90%. The lowest s/co ratio where TPHA was positive, was 3.1. Although RPR predicted > 95% of positive samples with s/co ratios > 15, its sensitivity was 47.7%. Conclusion: Higher s/co ratios can be used to define true-positivity and may indicate an active infection. TPHA may replace WB to confirm samples with s/co ratios between 3 and 15. RPR should not be used as a screening test in blood banks as it could miss almost half of the true-positive samples.