Background: Gray zone samples with optical density (OD) lying between cutoff OD and 10% below the cutoff OD (cutoff OD × 0.9) were identified during routine Transfusion Transmissible Infections (TTI) screening. Enzyme-linked immunosorbent assay (ELISA) used for this purpose can sometimes fail to detect blood donors who are recently infected or possessing the low viremia. Estimation of a gray zone in ELISA testing and repeat testing of gray zone samples can further help in reducing the risks of TTI in countries where nucleic acid amplification testing for TTIs is not feasible. Methods: Repeat ELISA testing was done on gray zone samples in duplicate, the samples showing both OD values below gray zone were marked nonreactive, and samples showing one or both OD values in the gray zone were marked indeterminate. The samples on repeat testing showing one or both OD above cutoff values were labeled reactive. Results: Out of the 196 gray zone samples, 33 samples turned out to be reactive (as shown in the table), 39 samples were nonreactive, and 124 samples were again in the gray zone and were labeled as indeterminate. Conclusion: Estimation of gray zone samples with repeat testing can further enhance the safety of blood transfusion in resource-poor developing nations where more sophisticated and sensitive methods such as nucleic acid amplification test (NAT) is not available in all the blood banks.

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