Patients receiving high dose ACV (> 2000 mg total oral daily dose or > 5 mg/kg IV three times daily) or vACV (Valtrex; > 3000 mg total daily dose) at the time of dosing. [Note: These doses are for patients with normal renal function; patients who dose reduce on the FDD are not excluded from the study.] 7. Patients with active CMV disease diagnosed within 6 months prior to enrollment; patients with CMV DNAemia requiring intervention with antiviral therapy at the time of enrollment.