HIV-infected children who are currently on TDF-base once daily regimen will be enrolled into this study. PK visit will be performed at least 1 month after changing regimen to once daily regimen. Adherence 3 days before PK study will be confirmed using adherence questionnaire. 100% of adherence is needed for PK study. If there is adherence problem on PK visit, PK study will be deferred to at least 3 days. On PK visit, blood drawn will be performed at 0, 2, 4, 6, 8, 10, 12, 24 hr. after taking ARVs for PK study. Blood drawn for safety evaluation will be drawn at any time-point on the PK visit. There is no study drug in this study. All ARVs that patient taking will be according to patient's requirement, and physician judgment. ARVs dosage will follow the Thai national pediatric guideline. CBC, CD4, viral RNA, viral resistance, ALT, cholesterol, and triglyceride results can be from other visits within 4 weeks window period.