Identification of the most clinically useful skeletal muscle mass indices pertinent to sarcopenia and physical performance in chronic kidney disease.

Aim: Patients with chronic kidney disease (CKD) are characterised by low skeletal muscle mass that negatively impacts physical performance. Operational definitions of 'low muscle mass' are inconsistent, and it is unknown how different skeletal muscle mass indices affect the relationship between muscle mass and physical function.

Methods: Appendicular skeletal muscle mass (ASM) was measured by dual-energy X-ray absorptiometry in 72 CKD patients. Along with crude ASM, alternative muscle indices were calculated adjusting for height, height-squared, body mass, and BMI. Physical performance was assessed by handgrip strength, sit-to-stand tests, gait speed, the incremental shuttle walk test and 'Short Physical Performance Battery'.

Results: Prevalence of 'low muscle mass' ranged from 26% to 35% of patients depending on the criteria used. The relationship between muscle mass indices and physical function differed for each criteria. Using average coefficients, the association with overall physical function and muscle indices were as follows: crude ASM (r = .258), ASM/height (r = .249), ASM/height-squared (r = .332), ASM/body mass (r = .249) and ASM/BMI (r = .206). Muscle adjusted for markers of adiposity (ASM/body fat %, r = .266; ASM/fat mass, r = .338) provided the best overall associations with physical function.

Conclusion: The use of alternative muscle mass indices provide different estimates of 'low muscle mass' prevalence, and the strongest (and most useful definition in regard to functional status) involves adjustment for either total or relative body fat. ASM adjusted for adiposity may be physiologically and clinically more relevant in patients with renal disease.