Analysis of sample-to-cutoff ratios on chemiluminescent immunoassays used for blood donor screening highlights the need for serologic confirmatory testing.

BACKGROUND: High sample-to-cutoff (s/co) ratios on hepatitis C virus antibody (anti-HCV) screening immunoassays (IAs) are indicative of confirmed-positive results and, according to some reports, can be used to determine anti-HCV status without the need for confirmatory testing. The purpose of this study was to determine whether s/co ratios on hepatitis B surface antigen (HBsAg), antibody to human immunodeficiency virus Types 1 and 2 (anti-HIV-1/2), anti-HCV, and antibody to human T-lymphotropic virus Types I and II (anti-HTLV-I/II) chemiluminescent immunoassays (ChLIAs) can be used to discriminate between biologic false-reactive (BFR) and confirmed-positive results.

STUDY DESIGN AND METHODS: In a blood donor population the s/co ratio distributions for BFR and confirmed-positive results were compared for the Abbott PRISM HBsAg, HIV O Plus, HCV, and HTLV-I/II ChLIAs to determine the extent of overlap between the two distributions for each assay.

RESULTS: The s/co ratio distributions for BFR and confirmed results overlapped in the range of 10.00 to 60.00, 1.00 to 6.00, 3.00 to 15.00, and 1.00 to 100.00 for the PRISM HIV O Plus, HCV, HTLV-I/II, and HBsAg assays, respectively.

CONCLUSION: Although high s/co ratios were predictive of confirmed-positive results in all four assays, a number of confirmed-positive samples gave low values while some biologic false-positive samples showed high values. As the s/co ratio distributions for BFR and confirmed-positive results overlapped for all four PRISM assays, this study highlights the importance of serologic confirmatory testing and the need for caution when using screening IA results to assign a final donor status.