The purpose of this study was to investigate the intra-assay correlations amongst initial reactive and repeat screening results used in enzyme immunoassays (EIAs) for hepatitis B virus (HBV), hepatitis C virus (HCV) and HIV in blood donors. This study evaluated the value of using the power of the signal to cut-off (S/CO) ratio index for confirming anti-HCV/HIV reactive screening results, thereby touching upon the utility of S/CO indices in determining whether further confirmatory testing was necessary. Screening test results of the 72 695 blood donors were evaluated over a 1-year period. Correlation analysis among each initial test and retests was done by Pearson r test. Appropriate S/CO values to determine the need of the confirmation testing was investigated by ROC analyses. EIA intra-assay correlations were of statistical significance and were determined as follows: 0 center dot 948 for anti-HCV, 0 center dot 827 for anti-HIV and 0 center dot 948 for HBsAg. The threshold S/CO ratio values which predicted more than 95% of the confirmation test result were 3 center dot 8 for HCV and 5 center dot 6 for HIV. We were able to demonstrate a strong level of intra-assay correlation amongst EIAs, thereby eliminating the need for repetition of the screening test. Hence, we suggest that repeat screening should only be limited to HBV and HIV tests with low EIA S/CO ratios. Thus, using the power of the S/CO ratio in determining the need for HCV confirmation testing can be a cost-effective measure, especially if the S/CO value is >= 3 center dot 8.