Creatinine Clearance Calculation Formulas
The Cockcroft-Gault formula is the most widely used method for estimating creatinine clearance: CrCl (ml/min) = [(140 - age) × weight (kg)]/[72 × serum creatinine (mg/dl)] × (0.85 if female). 1
Primary Formulas for Calculating Creatinine Clearance
Cockcroft-Gault Formula
- Estimated CrCl (ml/min) = [(140 - age) × weight (kg)]/[72 × serum creatinine (mg/dl)] × (0.85 if female) 1
- This formula was derived from a dataset of 249 men aged 18-92 years and assumes a 15% reduction in GFR for females 1
- For serum creatinine in μmol/l, divide by 88.4 to convert to mg/dl 1
Weight Considerations in Cockcroft-Gault
- For underweight patients: use actual body weight 2
- For normal weight patients: use ideal body weight 2
- For overweight, obese, and morbidly obese patients: adjusted body weight using a factor of 0.4 (ABW₀.₄) appears to be least biased 2, 3
- For obese patients: mean value between actual and ideal body weight is recommended 1
Alternative Formulas
MDRD (Modification of Diet in Renal Disease) Formula
- Estimated CrCl (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]) 1
- Provides GFR indexed to body surface area 1
- More accurate than Cockcroft-Gault in patients with significantly impaired renal function 1
Jelliffe Formula
- Estimated CrCl (ml/min/1.73 m²) = [98 - [0.8 × (age - 20)]] × [1 - (sex × 0.1)]/serum creatinine (mg/dl); sex = 0 if male and 1 if female 1
- Results expressed as ml/min × BSA/1.73 1
Wright Formula
- Estimated CrCl (ml/min) = [6550 - (38.8 × age)] × [1 - (0.168 × sex)] × BSA (m²)/serum creatinine (μmol/l); sex = 0 if male and 1 if female 1
Clinical Considerations
Limitations of Creatinine Clearance Estimation
- All formulas tend to be less accurate in elderly patients 1
- As renal function declines, tubular secretion and extrarenal elimination of creatinine increase, exaggerating the discrepancy between creatinine clearance and actual GFR 1
- Cockcroft-Gault underestimates GFR for normal and moderately reduced renal function but overestimates GFR in significantly impaired renal function 1
Serum Creatinine Measurement Methods
- The Jaffe method may overestimate serum creatinine by 5-15% compared to enzymatic methods 1
- When using peroxidase antiperoxidase (PAP) method, add 0.2 mg/dl to serum creatinine when using Cockcroft-Gault equation 1
- For standardized creatinine measurements that have replaced Jaffe in many institutions, divide serum creatinine by a factor of 0.95 1
Population-Specific Considerations
- For children over 12 years of age, Cockcroft-Gault can be used but should be considered as GFR corrected for body surface area 4
- For children under 12 years, no formula is satisfactory 4
- In African Americans, muscle mass represents on average 32.5% of total body weight versus 28.7% for white subjects, resulting in higher serum creatinine levels 1
Accuracy Comparison
- Despite requiring more time and effort, neither 24-hour urine collection nor timed creatinine clearance offers increased precision over calculation based on serum creatinine, sex, age, and weight in predicting GFR 5
- The original Cockcroft-Gault formula showed a correlation coefficient of 0.83 between predicted and measured creatinine clearance values 6
Remember that creatinine clearance provides only a crude measure of renal function and tends to overestimate true GFR because creatinine is both filtered and secreted by the kidneys 1.