Optimal acyclovir dosing under continuous renal replacement therapy (CRRT) in neonates is unknown. We monitored serum acyclovir levels and herpes simplex virus 1 (HSV-1) DNA levels in a neonate with disseminated HSV-1 infection and renal failure undergoing CRRT. A full-term, 5-day-old female presented with a 2-day history of lethargy and fever. She developed fulminant hepatitis and was diagnosed with HSV-1 infection by real-time polymerase chain reaction. Acyclovir was initiated at 60 mg/kg/day, which was lowered to 20 mg/kg/day because of development of renal failure. She was placed on continuous hemodialysis. Acyclovir dosing was adjusted according to serum acyclovir levels, and HSV-1 viral load was sequentially monitored. Semiquantification of serum HSV-1 levels was performed by real-time polymerase chain reaction. Acyclovir levels were measured by using liquid chromatography-tandem mass spectrometry. Acyclovir was administered at 20 mg/kg intravenously over 1 hour; peak concentration was 18.9 μg/mL. The half-life of acyclovir was estimated to be 2 to 3 h. Viral load remained high during dosing every 24 hours, with a decline of 0.17 log copies/24 hours. Acyclovir dosing was changed to 20 mg/kg/dose every 8 hours, with an average viral load decline of 0.44 log copies/24 hours. Despite the guideline recommendation of 24-hour redosing, acyclovir was dialyzed at a rate that resulted in suboptimal treatment. Individual therapeutic drug monitoring for acyclovir and dosing adjustment may be required to optimize therapy for patients undergoing CRRT.

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