CYP2B6 and hepatitis B virus infection: Univariate linear regression analysis identified the following variables as predictors (r2, p value) of efavirenz plasma level at week 16; CYP2B6*6 genotype (17.9%, p<0.0001), country (2.5%, p<0.0001), coinfection with Hepatitis B at recruitment (2.6%, p = 0.003).