Two Diagnostic Challenges in Hepatitis B Serology: Low HBsAg S/CO Values and Isolated Anti-HBc Positivity

Hepatitis B surface antigen (HBsAg) signal-to-cutoff (S/CO) values and isolated anti-HBc positivity represent two significant diagnostic challenges in hepatitis B virus (HBV) serology. This retrospective study, conducted in Karabuk between 2021 and 2025, aimed to evaluate the correlation between these profiles and HBV DNA positivity to determine the optimal S/CO thresholds for confirmatory testing and clarify the clinical relevance of isolated anti-HBc reactivity. Among 17,356 patients screened for anti-HBc, isolated anti-HBc positivity was identified in 506 patients (2.9%), of whom 3.2% had detectable HBV DNA. Positivity was significantly higher in patients older than 50 years and in the Gastroenterology department. Separately, 213 specimens with HBsAg S/CO values between 1.0 and 10.0 underwent HBV DNA confirmation testing. ROC analysis yielded an excellent area under the curve of 0.937, with an S/CO threshold of ≥3.04, achieving 100% sensitivity for HBV DNA detection. All specimens with S/CO values between 1.0 and 3.0 were HBV DNA-negative. In this study population, no serum HBV DNA positivity was detected among samples with S/CO values below 3.04, suggesting that HBV DNA testing may help avoid unnecessary confirmatory work-up in this range. Anti-HBc screening should be prioritized in older patients. S/CO-based confirmatory algorithm using HBV DNA testing may improve diagnostic accuracy and reduce unnecessary clinical interventions in routine hepatitis B screening programs.