Retrospective studies have shown that relapsers and non-responders to antiviral treatment with RBV had lower RBV levels than those who had a SVR. In a retrospective analysis of 4 studies investigating a total of 1105 patients treated with RBV, RBV plasma concentrations measured at 4 weeks of treatment correlated with viral clearance: SVR was 31.8% in those with RBV levels <1,000ng/ml, and increased to 62.5% with RBV concentrations at >4,000ng/ml.