Used as a stable isotope-labeled internal standard in mass spectrometry-based bioanalytical studies to accurately quantify telbivudine in biological samples such as plasma, serum, or urine. Its primary application is in pharmacokinetic and metabolism studies during drug development and clinical research. The deuterium-labeled structure provides a distinct mass difference from the unlabeled compound, allowing for precise measurement without interference. It is essential for validating analytical methods and ensuring reliable data in studies assessing drug exposure, bioavailability, and clearance.