Diagnostic power of serum creatinine/cystatin C ratio for identifying low MRI-muscle volume and low grip strength: Data from 9,731 to 149,707 UK Biobank older adults.

Background: Biomarkers for sarcopenia are lacking. We examined the diagnostic power of serum creatinine to cystatin C (Cr:Cyc) ratio for identifying low MRI-muscle volume and low grip strength in a large observational study of UK Biobank older adults.

Methods: Serum creatinine and cystatin C were measured via immunoassays (Beckman Coulter AU5800 and Siemens Advia 1800, respectively) and grip strength by hydraulic hand dynamometer at baseline visit (2008-2010). MRI-thigh fat-free muscle volume (FFMV) and DXA-derived appendicular lean mass were measured at imaging visit (2014-2018). Extreme outliers were removed, and covariates (demographic, lifestyle, and clinical factors, as well as time elapsed between baseline-imaging visit) were adjusted for in statistical models.

Results: 12,873 older adults (mean age: 63.5 ± 2.7 years, 44.2% women) were included for FFMV and ALM/BMI; 149,707 older adults (mean age: 64.0 ± 2.9 years, 50.5% women) for grip strength. Despite significant associations (p<0.05), in fully-adjusted models, Cr:Cyc showed poor to acceptable diagnostic power for identifying low FFMV when using cutpoints of 20th percentile (AUC: 0.577 men; 0.622 women) and T scores of -2 (AUC: 0.596 men; 0.659 women) and -2.5 (AUC: 0.609 men; 0.722 women). In fully-adjusted model, Cr:Cyc showed poor diagnostic power (AUCs: <0.70) for identifying low ALM/BMI or low grip strength irrespective of the cutpoint used.

Conclusions: Cr:Cyc may not be a suitable biomarker for identifying low muscle volume or low strength in older adults. This finding, drawn from a large sample size and the use of advanced medical imaging, marks an important contribution to the sarcopenia field.