This is an observational, 48-week prospective study of HIV-infected individuals treated with TFV in which the investigators will compare DBS TFV-DP levels in virologically suppressed vs. non-suppressed individuals and evaluate the utility of TFV-DP in DBS to predict virologic failure and also drug toxicity. To accomplish this, the investigators will approach HIV-infected patients currently taking TFV (which is being prescribed by a primary care physician) and who present to the clinic for regular HIV care. After informed consent is obtained, the investigators will collect extra blood samples for DBS TFV-DP and obtain information on drug adherence. The investigators will also collect extra blood samples for DBS TFV-DP at each subject's subsequent visit for approximately 3 visits in a 48 week period of time.