Objective To analyze the confirmation results of initial screening positive for human immunodeficiency virus (HIV) antibodies and to investigate the rationality of the gray zone for the signal-to-cutoff (S/CO) ratio values obtained by chemiluminescence assay.

Methods A total of 184 specimens with positive HIV antibody screening results were collected. Confirmation was conducted using the Western blot (WB) method and viral nucleic acid testing, and the confirmation outcomes and band distribution were observed. The receiver operating characteristic (ROC) curve was employed to predict the optimal cutoff value, and the rationality of S/CO values in establishing a gray zone for HIV antibody was analyzed.

Results Among the 184 HIV antibody screening positive samples, 153(83.15%) were initially screened by chemiluminescence, and 31 (16.85%) were screened by colloidal gold method. Male cases (57.07%) and those aged 40 years or older (75.00%) had higher proportion of initial screening positivity than females and other age groups. Supplemental WB testing revealed that 101 (54.89%) cases were confirmed as HIV-1 antibody positive, 56(30.43%) were negative, and 27(14.67%) were indeterminate. Of the 27 indeterminate cases, one was confirmed positive by NAT (initial S/CO value by chemiluminescence5.60), while the remaining 26 were negative. The positive agreement rates between chemiluminescence, colloidal gold and supplemental WB testing were 65.36% and 3.23%, respectively, with statistically significant difference (χ²=40.191, P < 0.05). Among 27 cases with indeterminate HIV-1 antibody results, upon further nucleic acid testing for viral load, one case exhibited WB banding pattern of P24+gp160, and the nucleic acid load confirmed this case as positive. In the distribution of band patterns among indeterminate samples, P24 had the highest occurrence (74.07%), followed by P66 (11.11%), gp160 (7.41%) as well as P17 and gp41 (both 3.70%). The ROC curve predicted the optimal S/CO cutoff value to be 5.15, with an area under the curve (AUC) of 0.904, sensitivity and specificity of 83.30% and 76.40%, respectively, and 95%CI of 0.861 to 0.948. There was a good correlation between the WB band patterns and confirmation results for initial screening positive cases (P < 0.05), with the highest correlation found for the P24 band (r=0.910).

Conclusion There is a certain rate of false positives among HIV antibody initial screening positive cases, and effective screening can be achieved through WB testing. Additionally, based on the current testing platform, the gray zone for HIV antibody S/CO values in the chemiluminescence assay is set at 1.00 to 5.15, which exhibits high sensitivity.