Calculating Renal Function

Introduction to Renal Function Calculations

The Cockcroft-Gault formula is recommended for calculating creatinine clearance, which is CrCl (mL/min) = [(140 - age) × weight (kg)] / [72 × serum creatinine (mg/dL)], multiplied by 0.85 for females, as suggested by the European Heart Rhythm Association 1, 2
The American College of Clinical Pharmacy recommends the Cockcroft-Gault equation for drug dosing purposes, particularly for renally-cleared medications with narrow therapeutic windows 1
The American Society of Clinical Oncology also recommends the Cockcroft-Gault equation for drug dosing purposes 1

The MDRD formula provides an alternative calculation method, which is Estimated GFR (mL/min/1.73 m²) = 186 × [serum creatinine (mg/dL)]^-1.154 × [age (years)]^-0.203 × [0.742 if female] × [1.21 if African American], as suggested by the National Kidney Foundation 2, 3, 4
MDRD is more accurate when GFR is less than 90 mL/min/1.73 m², and is superior in patients with significantly impaired renal function (stages 3-5 chronic kidney disease) 2, 3, 4

For obese patients, the European Heart Rhythm Association recommends using the mean value between actual body weight and ideal body weight rather than actual weight alone 1, 2
The MDRD equation has not been validated and should be avoided in normal persons or GFR >60 mL/min/1.73 m², diabetic kidney disease, patients with serious comorbid conditions, persons older than 70 years of age, extremes of body size, diseases of skeletal muscle, paraplegia or quadriplegia, vegetarian diet, and rapidly changing kidney function 1, 2, 3, 4

The Jaffe method overestimates serum creatinine by 5-15% compared to enzymatic methods, and the National Kidney Foundation recommends adding 0.2 mg/dL to the serum creatinine value to avoid underdosing when using enzymatic methods 1, 2

Creatinine clearance overestimates true GFR because creatinine is both filtered and secreted by the kidneys 1, 2
The Cockcroft-Gault formula underestimates GFR in normal and moderately reduced renal function but overestimates GFR when renal function is significantly impaired 2

Direct measurement using clearance methods is necessary in extremes of age and body size, severe malnutrition or obesity, diseases of skeletal muscle, paraplegia or quadriplegia, vegetarian diet, rapidly changing kidney function, and calculation of doses for potentially toxic drugs that are renally excreted 1, 2, 3, 4

African Americans have higher average muscle mass, resulting in higher baseline serum creatinine levels, and the MDRD formula accounts for this with a 1.21 multiplication factor 2

Serum creatinine concentration should never be used as the sole means to assess kidney function, and minor elevations in serum creatinine may represent substantial reductions in GFR, particularly in elderly patients, women, and those with lower muscle mass 3