Medication dosing in adult patients with reduced lean body mass and kidney injury: A focus on cystatin C

Purpose Creatinine-based estimates of glomerular filtration rate (GFR) have been the standard for classifying kidney function and guiding drug dosing for over 5 decades. There have been many efforts to compare and improve different methods to estimate GFR. The National Kidney Foundation recently updated the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations without race for creatinine (CKD-EPIcr_R) and creatinine and cystatin C (CKD-EPIcr-cys_R), and the 2012 CKD-EPI equation based on cystatin C (CKD-EPIcys) remains. The focus of this review is to highlight the importance of muscle atrophy as a cause for overestimation of GFR when using creatinine-based methods. Summary Patients with liver disease, protein malnutrition, inactivity, denervation, or extensive weight loss may exhibit markedly lower creatinine excretion and serum creatinine concentration, leading to overestimation of GFR or creatinine clearance when using the Cockcroft-Gault equation or CKD-EPIcr (deindexed). In some cases, estimated GFR appears to exceed the physiological normal range (eg, >150 mL/min/1.73 m2). Use of cystatin C is recommended when low muscle mass is suspected. One would expect discordance between the estimates such that CKD-EPIcys < CKD-EPIcr-cys < CKD-EPIcr ≈ Cockcroft-Gault creatinine clearance. Clinical evaluation can then occur to determine which estimate is likely accurate and should be used for drug dosing. Conclusion In the setting of significant muscle atrophy and stable serum creatinine levels, use of cystatin C is recommended, and the resulting estimate can be used to calibrate interpretation of future serum creatinine measurements.