Antiretroviral drug exposure is directly linked to individual host factors which include age, weight, diet, and genetics. However, the main factor impacting long-term drug exposure is drug adherence. Adherence is a strong predictor of HIV treatment outcomes, but measuring adherence is difficult due to the inaccuracy of self-reporting and other commonly used monitoring methods. To date, no gold standard measure to monitor antiretroviral exposure and adherence has been applied in clinical practice. Tenofovir (TFV) and its active metabolite, tenofovir diphosphate (TFV-DP), have distinctive pharmacological characteristics that make them ideal candidates for drug adherence and exposure monitoring. The long half life (~14-17 days) of TFV-DP in red blood cells (RBC) are properties well suited for monitoring average dose exposure over time. Based on these, the investigators propose that RBC levels of TFV-DP are an accurate and precise measure of long-term drug exposure in HIV-infected individuals. In addition, the investigators aim to quantify TFV-DP in dried blood spots (DBS) as a simple method to measure drug exposure.