Background: Confirmatory hepatitis B surface antigen (HBsAg) is an assay used to distinguish weakly reactive from false-positive HBsAg results. Objective: To determine the signal to cutoff (S/CO) value of chemiluminescence microparticle immunoassay (CMIA) HBsAg assay that should trigger follow-up confirmatory HBsAg testing. Methods: All specimens with an initial S/CO value of 0.90-100.00 were subjected to repeat HBsAg testing after high-speed centrifugation. The specimens with an initial S/CO value in that range remained in the same range and were then followed up with confirmatory HBsAg testing. Result: In total, 132 specimens had an S/CO value between 0.90 and 100.00 after high-speed centrifugation, followed by confirmatory HBsAg retesting. The S/CO value of HBsAg specimens for which the results required verification with confirmatory HBsAg was 0.98 (100% sensitivity, 3.3% specificity) through 9.32 (47.1% sensitivity, 100% specificity). Conclusion: The HBsAg S/CO values (as determined by the chemiluminescent microparticle immunoassay [CMIA] method) that should trigger confirmatory HBsAg testing are 0.98-9.32. © 2022 The Author(s). Published by Oxford University Press on behalf of American Society for Clinical Pathology. All rights reserved.