Multicenter retrospective chart review of patients admitted to any of the six study centers (SSM Health - St. Clare, St. Mary's, Saint Louis University Hospital, St. Anthony's, St. Mary's Madison, Rush University Medical Center, or Methodist Dallas Medical Center) between January 1, 2013 and December 31, 2019 will be conducted. All study centers utilize Epic ® electronic health record (Verona, Wisconsin; www.epic.com) for which all data will be extracted from. Each study center will obtain individual IRB approval prior to data collection. SSM - St. Clare will serve as the lead center for the study with all other centers sending collected and de-identified data to this central site for analysis.

• Obese - WHO definition: Body Mass Index > 30 kg/m2
• Ideal Body Weight (IBW) (male) - 50.0 kg + 2.3 kg x (patient height in inches, above 60 inches)
• Ideal Body Weight (female) - 45.5 kg + 2.3 kg x (patient height inches, above 60 inches)
• Adjusted Body Weight - IBW + [0.4 x (ABW - IBW)]
• HSV encephalitis - PCR positive for HSV-1 or HSV-2 in CSF during index admission
• Baseline Serum Creatinine (S Cr) - The SCr value obtained before the start of acyclovir therapy on which the dose of acyclovir should be based upon
• Acute Kidney Injury (AKI) - per RIFLE criteria and while receiving acyclovir: SCr doubles or urine output < 0.5 mL/kg/hour for 12 hours
• End-stage renal disease (ESRD) on admission - mention of a diagnosis of ESRD in electronic medical records prior to receipt of acyclovir
• Guideline-approved, weight-based doses - within 10% of 10 mg/kg dose (to account for rounding due to vial size)
• Adequate hydration - IV fluid order (other than KVO [keep vein open]) within 24 hours before acyclovir start AND either IV fluid order (other than KVO) for majority of acyclovir course or IV fluid + oral diet for majority of acyclovir course
• Appropriately renally dose-adjusted doses - Frequency of acyclovir matches package insert recommendations based on creatinine clearance at baseline
• Interacting medications - Mycophenolate, probenecid, zidovudine, tenofovir, valproic acid, phenytoin

Inclusion Criteria:

• Patients Admitted to a SSM Health - St. Clare (STL), St. Mary's (STL), Saint Louis University Hospital (STL), St. Anthony's (OKC), St. Mary's Madison (WIS), Rush University Medical Center (RUMC), or Methodist Dallas Medical Center
• > 18 years old
• Positive cerebrospinal fluid (CSF) PCR for HSV-1 or HSV-2
• BMI > 30 kg/m2
• Received guideline-approved, weight-based doses of IV acyclovir for HSV encephalitis for at least 72 hours during the index admission
• Received appropriately renal dose-adjusted doses of IV acyclovir per package insert recommendations

Exclusion Criteria:

• More than one dosing weight strategy utilized for > 24 hours for IV acyclovir dosing during the index admission
• Patients transferred to the hospital already on IV acyclovir for HSV encephalitis
• ESRD on admission
• Concomitant pathogen causing meningitis or encephalitis
• Oral acyclovir used for treatment

• More than one dosing weight strategy utilized for > 24 hours for IV acyclovir dosing during the index admission
• Patients transferred to the hospital already on IV acyclovir for HSV encephalitis
• ESRD on admission
• Concomitant pathogen causing meningitis or encephalitis
• Oral acyclovir used for treatment