CD4 and AIDS: Another study limitation is the possibility of a number of unknown confounders not taken into account in the analysis: despite adjustment for measurable variables (ie, sociodemographic characteristics, cumulative duration of antiviral exposure, nadir CD4 count, zenith HIV RNA, HIV viral subtype, number of previous VFs, RAMs, hepatitis C virus serostatus, previous AIDS-defining event), important covariates, such as adherence to antiretroviral therapy, could not be explored and accounted in the analysis.