Hong Life CalculatorsUnderstanding the creatinine clearance (Cockcroft-Gault) calculator — concept and principle
Creatinine clearance (CrCl) estimates how many millilitres of blood plasma the kidneys clear of creatinine each minute. The Cockcroft-Gault equation calculates it from age, body weight, sex and serum creatinine. It remains the formula most drug labels refer to for dose adjustment, so pharmacists and doctors still use it alongside eGFR.
How it is calculated
CrCl (mL/min) = (140 − age) × weight (kg) ÷ (72 × serum creatinine in mg/dL), multiplied by 0.85 for women.
Age is in years, weight in kilograms and creatinine in mg/dL. The result is not scaled to body surface area, so it describes the individual's actual clearance.
The principle
Clearance compares how fast a substance leaves the blood with how much is in it. Creatinine production depends on muscle mass, which rises with body weight and falls with age, so Cockcroft and Gault (1976) put weight in the numerator and (140 − age) to model the age decline. Dividing by serum creatinine reflects the inverse relationship: the higher the blood level, the lower the clearance. The constant 72 calibrates the units, and the 0.85 factor for women reflects lower average muscle mass. The original data came from 249 adult men, so the women's factor was an assumption rather than a measured value.
Worked example
- Inputs: male, age 35, weight 75 kg, creatinine 1.0 mg/dL.
- Numerator: (140 − 35) × 75 = 105 × 75 = 7,875.
- Denominator: 72 × 1.0 = 72.
- CrCl = 7,875 ÷ 72 = 109.375, shown as 109 mL/min (no female factor).
| Serum creatinine (mg/dL) | CrCl (mL/min) |
|---|---|
| 0.25 | 438 |
| 0.5 | 219 |
| 0.75 | 146 |
| 1 | 109 |
| 1.25 | 88 |
| 1.5 | 73 |
| 2 | 55 |
How to read the result
Values around 100 mL/min or above are typical for healthy young adults, and clearance falls naturally with age. Drug labels often use bands such as above 50, 30–50 and below 30 mL/min to set reduced doses, but the exact cut-offs differ by medicine, so always check the specific label. Because the formula uses actual weight, it can overestimate clearance in people with obesity; clinicians may use ideal or adjusted body weight instead.
Common mistakes
- Entering creatinine in µmol/L; divide by 88.4 first.
- Comparing CrCl directly with eGFR, which is scaled to 1.73 m² and uses a different equation.
- Using the formula when creatinine is changing quickly, for example in acute kidney injury.
Frequently asked questions
Should I use actual or ideal body weight?
The original equation used actual weight. For people well above ideal weight, many clinicians use ideal or adjusted weight to avoid overestimating clearance.
Why is creatinine clearance higher than measured GFR?
The kidney tubules also secrete a small amount of creatinine, so creatinine-based clearance tends to slightly overestimate true filtration.
Was the creatinine assay a factor?
Yes. The equation predates standardised (IDMS-traceable) creatinine methods, which give slightly lower values, so modern results may differ a little from the original study.
Sources: Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron 1976;16(1):31–41.