Very low hepatitis C antibody levels predict false-positive results and avoid supplemental testing

False-positive results for hepatitis C virus antibody (anti-HCV) occur with unacceptable frequency in low-prevalence populations. The purpose of the study was to determine whether signal-to-cutoff (S/CO) ratios of anti-HCV assay-reactive samples could be used to discriminate false-positive from true-positive anti-HCV results and avoid the need for supplemental testing.

Using receiver-operating characteristic curve, the cutoff point that identifies the major proportion (>/=95%) of false-positive results, with a minor proportion (<5%) of true-positive anti-HCV results, was determined. An anti-HCV assay (VITROS, Ortho Clinical Diagnostics) was used to detect the antibodies. The third-generation recombinant immunoblot assay and HCV RNA tests were performed on all included donors. Third-generation RIBA is the gold standard for identifying false-positive antibody results.

A total of 649 anti-HCV-positive blood donors were identified. A S/CO ratio of less than 4.5, defining very low levels in this value, was the optimal cutoff point to identify false-positive results; 315 of 322 samples with very low levels were false-positive anti-HCV results (97.8%; 95% confidence interval [CI], 95.8%-99.0%) and 7 were true-positive (2.2%; 95% CI, 1.0%-4.3%). Viremia was detected in none of them. A direct relationship was observed between positive supplemental testing and increased antibody levels in the other 327 samples.

The high prediction rate of false-positive anti-HCV results using very low levels by the Ortho VITROS anti-HCV assay safely avoids the need for supplemental testing.