GSTM1 and anemia (phenotype): Inter-patient variability in the severity of anemia may arise from variations in the intracellular uptake, accumulation, or metabolism of ribavirin, as well as in the dependence of the RBC lifespan on ribavirin accumulation and in the sensitivity of the RBC production rate to changes in Hb. To obtain a measure of this inter-patient variability, we calculated  using 500 different combinations of the values of the parameters, , ,  and , for a range of values of .