From the Guidelines
Creatinine clearance is best calculated using the Cockcroft-Gault formula, which estimates kidney function based on serum creatinine, age, weight, and gender, as recommended by the 2018 European Heart Rhythm Association practical guide 1. To calculate creatinine clearance, multiply (140 - age in years) by weight in kilograms, then divide by 72 times serum creatinine in mg/dL. For females, multiply the result by 0.85 to account for lower muscle mass.
Steps to Calculate Creatinine Clearance:
- Multiply 140 by the patient's weight in kilograms
- Subtract the patient's age from the result
- Divide the result by 72 times the serum creatinine in mg/dL
- For females, multiply the result by 0.85 For example, for a 60-year-old male weighing 70 kg with a serum creatinine of 1.2 mg/dL, the calculation would be: [(140 - 60) × 70] ÷ (72 × 1.2) = 80 mL/min.
Important Considerations:
- You'll need a recent serum creatinine test, accurate weight measurement, and the patient's exact age to perform the calculation.
- This calculation is essential for medication dosing adjustments in patients with kidney impairment, as noted in the study by Launay-Vacher et al. 1.
- The formula works because creatinine is produced at a constant rate from muscle metabolism and is primarily eliminated through glomerular filtration, making it a reliable marker of kidney function.
- Normal values range from 90-140 mL/min for men and 80-125 mL/min for women, with values below 60 mL/min indicating kidney disease, as outlined in the 2018 European Heart Rhythm Association practical guide 1.
Online Resources:
Online calculators, such as those found at www.kidney.org/professionals/kdoqi/gfr_calculator, and popular apps like NephroCalc, MedMath, MedCalc, Calculate by QxMD, and Archimedes, can also be used to estimate creatinine clearance 1.
From the Research
Calculating Creatinine Clearance
To calculate creatinine clearance, several formulas can be used, including:
- The Cockcroft-Gault equation, which estimates creatinine clearance and incorporates weight into the equation 2
- The Modification of Diet in Renal Disease and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations, which estimate glomerular filtration rate and incorporate ethnicity but not weight 2
Formulas for Calculating Creatinine Clearance
Some studies have developed formulas to predict creatinine clearance from serum creatinine, such as:
- A formula developed in 1976 for adult males: (see article) with a 15% reduction for females 3
- The Cockcroft-Gault equation, which has been shown to be accurate in various populations, including Nigerian patients 4
- A serum cystatin C-based equation, which has been proposed as a reliable marker of glomerular filtration rate 5
Factors Affecting Creatinine Clearance
Several factors can affect creatinine clearance, including:
- Age: factors for age must be included for reasonable prediction 3
- Body weight: weight is incorporated into the Cockcroft-Gault equation 2
- Sex: the formula developed in 1976 requires a 15% reduction for females 3
- Disease state: the level of creatinine concentration and disease state can affect the predictive value of the equation 4
Comparison of Equations
Different equations can give varying estimates of renal function, and the choice of equation can impact clinical decisions, such as dose adjustment for anticoagulation therapy 2. The Cockcroft-Gault equation has been shown to be more accurate than serum creatinine for assessing glomerular filtration rate 6, but may not be equal to the CKD-EPI Collaboration equation 2.