ITPA and anemia (phenotype): RBV-induced anaemia has been studied in patients with chronic hepatitis C virus infection.23,24 In one study, bodyweight and haemoglobin concentrations were shown to be relevant covariates in an indirect response model that did not include RBV plasma concentrations.23 In another model, plasma and intracellular RBV phosphorylation kinetics were linked to the effect of RBV triphosphate accumulation on red blood cell homeostasis.24 Relevant covariates in this model were sex, weight and inosine triphosphatase (ITPA) genotype.