Used as an internal standard in mass spectrometry for the accurate quantification of ganciclovir in biological samples such as plasma, serum, and urine. Enables precise monitoring of drug levels in pharmacokinetic studies and therapeutic drug monitoring, especially in patients undergoing antiviral therapy for cytomegalovirus (CMV) infections. Its deuterated structure allows clear differentiation from non-labeled ganciclovir during analytical testing, improving measurement accuracy and reliability in clinical and research settings.