Creatine (methyl-d3) dilution in urine for estimation of total body skeletal muscle mass-accuracy and variability vs. MRI and DXA

A noninvasive method to estimate muscle mass based on creatine (methyl-d₃) dilution (D₃-creatine) using fasting morning urine was evaluated for accuracy and variability over a 3-4 mo period. Healthy older (67-80 y) subjects (n=14) with muscle wasting secondary to aging and 4 patients with chronic disease (58-76 y) fasted overnight, then received an oral 30-mg dose of D₃-creatine at 8 am (day 1). Urine was collected during 4 h of continued fast then at consecutive 4-8-h intervals through day 5. Repeat assessment was performed 3-4 mo later in 13 healthy subjects and 1 patient with congestive heart failure. Deuterated and unlabeled creatine and creatinine were measured using liquid chromatography-tandem mass spectrometry. Total body creatine pool size and muscle mass were calculated from D₃-creatinine enrichment in urine. Muscle mass was also measured by whole-body MRI and 24-h urine creatinine, and lean body mass (LBM) by dual-energy x-ray absorptiometry (DXA). D₃-creatinine urinary enrichment from day 5 provided muscle mass estimates that correlated with MRI for all subjects (r=0.88, P<0.0001), with less bias (mean±SD difference from MRI: –3.00±2.75 kg) compared with total LBM assessment by DXA, which overestimated muscle mass vs MRI (+22.5±3.7 kg). However, intra-individual variability was high with the D₃-creatine dilution method, with intra-subject SD for estimated muscle mass of 2.5 kg vs MRI (0.5 kg) and DXA (0.8 kg). This study supports further clinical validation of the D₃-creatine method for estimating muscle mass.

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