Reply to Estimated Glomerular Filtration Rate and Muscle Mass in Older Patients: Diagnostic Accuracy of Creatinine-Based Equations and Implications in Practice

We thank doctors Dharmarayan and Yoo for their comments on our article. In line with the literature and our results, they found a substantial lack of consistency among the results of different formulas estimating renal function by investigating a sample including a large proportion of frail older people. We believe that this could partly depend on the age-composition of different samples generating such equations, which in turn may reflect differently the age-related decay of skeletal muscle mass. Interestingly, Dharmarajan et al found that the Cockcroft-Gault formula (C-G) provided worse estimate of kidney function than Modification of Diet in Renal Disease and Chronic Kidney Disease Epidemiology Collaboration equations in patients aged ≥70 years. In our opinion, the latter result should be handled with care. The National Kidney Foundation recommends against the clinical use of C-G because it was generated before standardized creatinine assays were available. C-G estimates creatinine clearance (CrCl) rather than glomerular filtration rate. CrCl is in turn affected by proximal tubular secretion of creatinine and the decline of renal function is associated with an augmented contribution of tubular creatinine secretion to the total CrCl by the residual nephrons. Moreover, weight being equal, lean mass decreases and fat mass increases with aging, so that the C-G (only including weight) may also suffer some inaccuracy arising from the age-related changes of body composition. In conclusion, we strongly agree with the final remark of the authors that glomerular filtration rate progression should always be estimated the same way in each patient to reliably compare the results. This seems quite trivial but reflects the real world clinical experience of the authors.