Measuring kidney filtration directly
Creatinine is a waste product of muscle that the kidneys filter out at a fairly steady rate. Comparing how much creatinine appears in a timed urine collection with the level in the blood gives the creatinine clearance, the volume of blood plasma cleared of creatinine each minute. It is a measured estimate of the glomerular filtration rate (GFR).
The formula
Clearance (mL/min) = urine creatinine × urine volume ÷ (blood creatinine × minutes)
The two creatinine values must be in the same unit. The calculator then normalizes the result to a standard body surface area of 1.73 m², using the Mosteller formula BSA = √(height in cm × weight in kg ÷ 3,600):
Normalized clearance = clearance × 1.73 ÷ BSA
Worked example
A 24-hour collection of 1,500 mL has a urine creatinine of 100 mg/dL, and blood creatinine is 1.0 mg/dL. The patient is 5 ft 10 in and weighs 200 lb.
| Result | Value |
|---|---|
| Clearance, not normalized | 104 mL/min |
| Body surface area | 2.12 m² |
| Normalized clearance | 85 mL/min/1.73 m² (G2) |
| Creatinine excreted | 16.5 mg/kg/day |
Interpreting the result
The normalized value is compared with the usual chronic kidney disease stages: G1 at 90 or more, G2 from 60 to 89, G3a from 45 to 59, G3b from 30 to 44, G4 from 15 to 29 and G5 below 15. A stage is only one part of the picture; protein in the urine and the trend over time matter too.
For drug dosing, many drug labels refer to the non-normalized clearance in mL/min, because the dose depends on the person’s actual kidney capacity rather than an average body size.
Checking the collection
The most common error is an incomplete collection. Adults usually excrete about 15 to 25 mg of creatinine per kg per day (men) and 10 to 20 (women). A much lower value suggests missed urine, which would make the clearance falsely low. To collect correctly: empty the bladder at the start time and discard that urine, then collect every drop until the same time the next day, including a final void.
When it is used
Estimated GFR equations based on blood creatinine are usually enough. A measured clearance is useful when the estimate is unreliable: very high or very low muscle mass, amputations, vegetarian diets or creatine supplements, liver disease, pregnancy, or before giving drugs with a narrow safety margin. Because some creatinine is secreted by the tubules, measured clearance tends to overestimate the true GFR by 10 to 20%.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.