Creatinine Clearance (Cockcroft-Gault)
Estimates kidney function to guide drug dosing adjustments.
Formula
Variables
Example
Age 60, 70 kg, Scr 1.2 → CrCl ≈ 65 mL/min
How it works
Cockcroft-Gault estimates creatinine clearance from age, body size, and serum creatinine because creatinine production is tied to muscle mass while kidney filtration removes creatinine from the blood. As kidney function falls, serum creatinine tends to rise, so the formula uses creatinine level in the denominator to convert that blood test into an approximate filtration rate. It is not a direct measurement of GFR, but it is widely used because many drug doses were originally studied and adjusted using this estimate.
When to use it
Use this when you need a quick estimate of renal function for medication dosing, especially for drugs that are cleared by the kidneys or have narrow therapeutic ranges. It is commonly used in adults when reviewing dose adjustments, although special caution is needed in very small, very obese, elderly, or unstable patients.
Worked example, step by step
Example: 45-year-old male, weight 80 kg, serum creatinine 1.0 mg/dL Step 1: Subtract age from 140: 140 - 45 = 95 Step 2: Multiply by weight: 95 × 80 = 7600 Step 3: Multiply serum creatinine by 72: 72 × 1.0 = 72 Step 4: Divide numerator by denominator: 7600 ÷ 72 = 105.6 Step 5: Male patient, so no 0.85 correction Final answer: CrCl ≈ 106 mL/min
Common mistakes to avoid
- Using the formula for children or for patients with rapidly changing kidney function
- Forgetting the female correction factor of 0.85
- Mixing up units, such as using serum creatinine in µmol/L instead of mg/dL without converting first
Did You Know?
The Cockcroft-Gault equation from 1976 is still used daily in hospitals worldwide despite being nearly 50 years old.
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IntermediateAverage plasma drug concentration when intake and elimination are balanced.
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