Statistical considerations Sample size calculations. There are no pharmacokinetic data in children. In adults, the mean (SD) AUC following administration of a 300mg Viread tablet in fasting state is 2.29 (0.69) microg.hr/mL. In patients taking lopinavir/ritonavir and atazanavir/ritonavir, the mean AUC of TDF is increased by 32% and 24% respectively. Efavirenz does not change the AUC of TDF. If the weight adjusted dose of TDF in children and adolescents achieves the same AUC as in adults and concomitant dosing with a protease inhibitor increases the TDF AUC by 30% but concomitant dosing with Efavirenz does not change the TDF AUC, then 20 treated with Efavirenz and 20 patients treated on a PI based regimen would provide 90% power to detect this difference at the 5% significance level.