Determination of Cut-Off Index of Reactive HBsAg Based on Confirmatory Test at Dr. Moewardi Hospital

Abstract

HBsAg confirmatory test is required to confirm false reactive HBsAg due to non-specific binding of antigen with antibody. HBsAg confirmatory test cannot be performed in every laboratory due to the high price of reagents, short reagent shelf life, prolonged turnaround time, and increasing complexity of the test. This study aimed to determine the HBsAg COI (cut-off unit) cut-off point, which requires a confirmatory HBsAg test on the COBAS e411 analyzer with the ECLIA method. An observational analytic study with a cross-sectional approach was performed on 59 patients who underwent the HBsAg test at Dr. Moewardi Hospital, Surakarta in September-October 2020 on the Cobas e411 analyzer with the ECLIA method. The best analytical performance of cut-off index of reactive HBsAg was determined using the ROC curve and AUC. The results of initial HBsAg COI showed the following results: borderline with a mean: 0.975±0.0014 and reactive with a median of 4.38 (1.03-10)). The confirmatory test showed 59.8% reactive, 22.2% non-reactive, 13.8% not valid and 4.2% indeterminate results. The cut-off value for reactive HBsAg was 4.34 with an AUC of 0.818 (95% CI: 0.713 - 0.923; p=0.054), a sensitivity of 72.1% and a specificity of 87.5%. The initial cut-off of HBsAg index at 4.34 COI showed the best analytical performance with a sensitivity of 72.1% and specificity of 87.3%. Therefore, it can be used to determine HBsAg results that require HBsAg confirmatory examination on the Cobas e411 analyzer with the ECLIA method.