Background: The hepatitis C virus core antigen (HCVcAg) test is an alternative to measuring hepatitis C virus (HCV) ribonucleic acid (RNA) levels to diagnose active HCV infection. Data are lacking regarding the correlation between HCVcAg and HCV RNA in HCV genotype 6 which are more prevalent in Southeast Asia than in other parts of the world.

Objective:This study aimed to evaluate the diagnostic performance of HCVcAg in detecting chronic hepatitis C virus (HCV) infection, the correlation and association between HCVcAg assay results and HCV RNA levels in Thai patients carrying each HCV genotype.

Materials and Methods: We prospectively enrolled 165 anti-HCV positive patients who visited Srinagarind hospital from October 2017 to November 2018 for pre-HCV treatment evaluations. All samples were tested using HCVcAg (Architect, Abbott Laboratories). The diagnostic performance of the HCVcAg test was assessed using receiver operating characteristic (ROC) curves to identify the best cutoff value for detection of HCV viremia. A Spearman's correlation and linear regression were used to assess the correlation and association between HCVcAg and HCV RNA.

Results: The most reliable HCVcAg cut-off for diagnosis HCV infection was HCVcAg >6 fmol/L, with a sensitivity of 95.3% (95% confidence interval [CI] 90.5-98.1%), specificity of 100% (95% CI 80.5-100%), positive predictive value of 100% (95% CI 97.4-100%), and negative predictive value of 70.8% (95% CI 48.9-87.4%). The AUROC was 0.976 (95% CI 0.955-0.997). There was a strong, statistically significant positive correlation between HCVcAg and HCV RNA in all genotypes (r = 0.8915, p< 0.001). The regression equation was log₁₀ HCV RNA = 1.3467 log₁₀ HCVcAg + 1.8318 (95% CI for slope and intercept, 1.2359-1.4574 and 1.5018-2.1619, respectively). The HCV RNA level corresponding to the cutoff of 6 fmol/L of the HCVcAg test was approximately 800 IU/mL.

Conclusion: The HCVcAg assay exhibited excellent diagnostic performance and a good correlation with HCV RNA in all genotypes. This assay can thus be used as a lower-cost alternative tool for diagnosis of active HCV infection in anti-HCV positive patients, allowing these patients greater accessibility to chronic HCV treatment.