Indicator Dilution TechniquesRadioisotope Dilution TechniqueMass SpectrometryGas Chromatography-Mass SpectrometryReproducibility of ResultsReference StandardsIsotopesChromatography, High Pressure LiquidTandem Mass SpectrometryIsotope LabelingSpectrometry, Mass, Electrospray IonizationCreatinineChromatography, LiquidMagnetic Resonance SpectroscopyNitrogen IsotopesOxygen IsotopesSpectrometry, Mass, Matrix-Assisted Laser Desorption-IonizationDeuteriumCarbon IsotopesCalibrationBody WaterLimit of DetectionProteomicsSensitivity and SpecificitySolid Phase Extraction