The purpose of this study is to evaluate cumulative exposure to tenofovir diphosphate (TFV-DP) using dried blood spots (DBS) in treated HIV-infected patients who are receiving a TFV-based regimen. Using DBS will allow the investigators to assess this simple method to measure drug exposure in the clinical setting. The investigators hypothesize that TFV-DP levels will be lowest in individuals with a detectable viral load and highest in those with viral suppression.