Creatinine Clearance Formula (Cockcroft-Gault)
Cockcroft-Gault equation estimates kidney function: CrCl = [(140-age) × weight / (72 × SCr)] × 0.85 if female.
Used for drug dosing in renal impairment.
The Formula
The Cockcroft-Gault equation estimates creatinine clearance (CrCl), a measure of kidney function, from routine blood tests. CrCl approximates the glomerular filtration rate (GFR) — how much blood the kidneys filter per minute.
Age is in years, IBW is ideal body weight in kg, SCr is serum creatinine in mg/dL. The 0.85 factor applies to females because women have less muscle mass and produce less creatinine than men of the same size. For obese patients, use adjusted body weight (IBW + 0.4 × (actual − IBW)).
Normal CrCl: 80–120 mL/min. Mild impairment: 60–80. Moderate: 30–60. Severe: 15–30. Kidney failure (dialysis needed): below 15.
Many drugs are eliminated by the kidneys and require dose adjustments when CrCl is reduced. Antibiotics (gentamicin, vancomycin), digoxin, metformin, and low-molecular-weight heparins all need renal dose adjustment. Metformin is contraindicated below CrCl 30 due to lactic acidosis risk.
Ideal body weight for males: 50 + 2.3 × (height in inches − 60). For females: 45.5 + 2.3 × (height in inches − 60).
Variables
| Symbol | Meaning | Unit |
|---|---|---|
| CrCl | Creatinine clearance (kidney function) | mL/min |
| age | Patient age | years |
| IBW | Ideal body weight | kg |
| SCr | Serum creatinine | mg/dL |
Example 1
Male, age 65, IBW 70 kg, SCr 1.4 mg/dL.
CrCl = (140 − 65) × 70 / (72 × 1.4) = 75 × 70 / 100.8
CrCl = 52.1 mL/min (moderate renal impairment — dose adjustment needed for many drugs)
Example 2
Female, age 45, IBW 60 kg, SCr 0.9 mg/dL.
CrCl = [(140 − 45) × 60 / (72 × 0.9)] × 0.85 = [5,700 / 64.8] × 0.85
CrCl = 74.8 mL/min (normal to mild)
When to Use It
- Dosing renally-cleared drugs in patients with impaired kidney function
- Deciding whether metformin, NSAIDs, or contrast dye are safe
- Monitoring progressive kidney disease in diabetic patients
- Clinical pharmacokinetics and nephrology practice