BackgroundThe concentration of circulating viral RNA at baseline and during antiviral therapy predicts response in both HIV infection and chronic hepatitis C. MethodsRetrospective review of longitudinal plasma HCV-RNA determinations in untreated chronic hepatitis C patients. As comparison, longitudinal plasma HIV-RNA measurements were performed in untreated HIV individuals. ResultsA total of 3169 HCV-RNA determinations over 43.0±31.6 months were available for 818 chronic hepatitis C patients. For 333 HIV individuals, 1998 HIV-RNA measurements were examined over 27.3±17.5 months.Overall 44% consecutive specimens had >0.5 log variation in HCV-RNA values compared to 23% for HIV-RNA (p<0.001). These rates were 15% and 4%, respectively, for variations >1 log (p<0.001). In multivariate analysis (odds ratio, 95% confidence interval, p), the likelihood of experiencing HCV-RNA variations >0.5 log IU/mL was greater in patients with lower HCV-RNA (0.35 per log[0.26–0.47], 0.001), HIV coinfection (2.57[91.56–4.23], <0.001) and IL28B-CC (1.87[1.28–2.74], 0.001). ConclusionsFluctuation in circulating HCV-RNA may be clinically meaningful in a substantial proportion of chronic hepatitis C patients. It may influence the best time to prescribe antiviral therapy. Moreover, decisions based on early viral kinetics, such as early stopping rules, may require testing of baseline specimens the closest to treatment initiation.