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

Abstract

A noninvasive method to estimate muscle mass based on creatine ( methyl-d₃) (D₃-creatine) dilution using fasting morning urine was evaluated for accuracy and variability over a 3- to 4-mo period. Healthy older (67- to 80-yr-old) subjects ( n = 14) with muscle wasting secondary to aging and four patients with chronic disease (58-76 yr old) fasted overnight and then received an oral 30-mg dose of D₃-creatine at 8 AM ( day 1). Urine was collected during 4 h of continued fasting and then at consecutive 4- to 8-h intervals through day 5. Assessment was repeated 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) was measured 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 [difference from MRI = -3.00 ± 2.75 (SD) kg] than total LBM assessment by DXA, which overestimated muscle mass vs. MRI (+22.5 ± 3.7 kg). However, intraindividual variability was high with the D₃-creatine dilution method, with intrasubject 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. NEW & NOTEWORTHY Measurement of creatine ( methyl-d₃) (D₃-creatine) and D₃-creatinine excretion in fasted morning urine samples may be a simple, less costly alternative to MRI or dual-energy X-ray absorptiometry (DXA) to calculate total body muscle mass. The D₃-creatine enrichment method provides estimates of muscle mass that correlate well with MRI, and with less bias than DXA. However, intraindividual variability is high with the D₃-creatine method. Studies to refine the spot urine sample method for estimation of muscle mass may be warranted.

Keywords: creatine; creatinine; dual-energy X-ray absorptiometry; lean mass; muscle mass.

Fig. 1.

Cumulative percentage of D₃-creatine dose excreted in urine by population: healthy men (n = 10), healthy women (n = 4), congestive heart failure (CHF) patients (n = 2 men), and chronic obstructive pulmonary disease (COPD) patients (n = 2 men). Values are means ± SD.

Fig. 2.

Observed cumulative D₃-creatine excretion vs. 96- to 100-h ratio of urine unlabeled creatine to creatinine (Cr/Crn) with fitted lines by sex.

Fig. 3.

Predicted vs. observed cumulative D₃-creatine excretion with reference line at y = x (n = 14 males and 4 females).

Fig. 4.

Urine D₃-creatinine enrichment vs. time by population: healthy men (n = 10), healthy women (n = 4), CHF patients (n = 2 men), and COPD patients (n = 2 men). Values are means ± SD.

Fig. 5.

Scatterplots of magnetic resonance imaging (MRI) vs. alternate methods with reference lines at y = x. A: MRI vs. D₃-creatine method 1 estimate of muscle mass. B: MRI vs. D₃-creatine method 2 estimate of muscle mass. C: MRI vs. D₃-creatine method 3 estimate of muscle mass. D: MRI vs. dual-energy X-ray absorptiometry (DXA) total lean body mass. E: MRI vs. DXA appendicular lean mass. F: MRI vs. 24-h urinary creatinine excretion estimate of muscle mass.