Signal-to-cutoff ratios of current anti-HCV assays and a suggestion of new algorithm of supplementary testing

The detection of hepatitis C virus antibody (anti-HCV) is known to have high false-positive rates. Using signal-to-cutoff (S/Co) ratios in reflex supplemental testing, however, could reduce false-positive rates. Here, we analyzed the 2-year data of an anti-HCV assay to understand the significance of the S/Co ratio and make a new algorithm by confirming with a second anti-HCV assay.

We reviewed 32,573 samples of the Architect assay (Abbott Diagnostics) from a tertiary hospital. Retests with the Elecsys (Roche Diagnostics) and Vitros (Ortho Clinical Diagnostics) assays were performed in 346 anti-HCV-positive samples. HCV RNA PCR and recombinant immunoblot assay (RIBA) were performed in 147 and 11 anti-HCV-positive samples, respectively.

Among 32,573 samples, 446 (1.37%) yielded positive results and 32,127 (98.6%) yielded negative results. Concordance rates in low S/Co samples (0.9-10.0) were 35.2%, 43.8%, and 81.9% for the Architect-Elecsys, Architect-Vitros, and Elecsys-Vitros comparisons, respectively. Correlation coefficients of S/Co ratios were as follows: Architect-Elecsys, 0.20; Architect-Vitros, 0.42; and Elecsys-Vitros, 0.46. In logistic regression, the S/Co value for predicting positivity with 95% probability was 3.13, while that for predicting 50% probability was 8.85. S/Co ratios of 1.70-3.34 showed one reactive antigen out of five antigens, and S/Co ratios of 13.54-17.72 showed three to five positive reactions out of five antigens used in the RIBA.

Supplementary testing of anti-HCV screening results is necessary to distinguish between true positivity and biological false positivity for anti-HCV. In this study, we presented an algorithm of supplementary testing by a retest with a second reagent, which could be useful in clinical laboratories.