Special populations including women with known pregnancy, prisoners or institutionalized individuals;Previous renal transplantation;Known acutely worsening renal function prior to enrollment (e.g., any category of RIFLE criteria. Already receiving dialysis (either acute or chronic) or in imminent need of dialysis at the time of enrollment; Known infection with human immunodeficiency virus (HIV) or active hepatitis (acute or chronic);Patient meets any of the following:Active bleeding with an anticipated need for > 4 units PRBC;Hemoglobin < 7 g/dL;Any other condition that in the physician's opinion would contraindicate drawing serial blood samples for clinical study purposes.