Measured 24h Urea & Creatinine Clearance Calculator
Calculate measured 24h creatinine and urea clearance. Get an accurate GFR estimation (Mean Urea/Creatinine Clearance) adjusted to body surface area (BSA).
Measured Creatinine and Urea Clearance: Accurate GFR Evaluation
In nephrology, estimating the Glomerular Filtration Rate (GFR) is commonly done via mathematical equations (CKD-EPI, MDRD). However, in certain patients (extreme malnutrition, morbid obesity, amputees, strict vegan diets), these empirical formulas are inaccurate. It then becomes essential to perform a measured clearance using a 24-hour urine collection.
Why calculate the Mean Urea/Creatinine Clearance?
Using creatinine clearance alone presents a major bias, especially in advanced chronic kidney disease:
- Creatinine overestimates GFR: A significant fraction of urinary creatinine is not only filtered by the glomeruli but also actively secreted by the renal tubules (causing a 20-30% overestimation).
- Urea underestimates GFR: Conversely, urea is filtered and then partially reabsorbed by the renal tubules.
- The Solution (Mean GFR): The arithmetic mean of creatinine clearance and urea clearance cancels out these two opposing biases. It is the recommended method for evaluating residual renal function in patients with stage 4 and 5 chronic kidney disease (pre-dialysis).
The Importance of BSA Indexing and Urine Volume
An incomplete urine collection is the most frequent source of error for this test. Furthermore, raw clearance results (in mL/min) must be indexed to the standard Body Surface Area (BSA) of 1.73 m² to be comparable with KDIGO classification standards. This calculator uses the Du Bois formula to automatically adjust the final result to mL/min/1.73m².
KDIGO Classification (Chronic Kidney Disease)
| GFR (mL/min/1.73m²) | KDIGO Stage | Description |
|---|---|---|
| ≥ 90 | G1 | Normal or high |
| 60 - 89 | G2 | Mildly decreased |
| 45 - 59 | G3a | Mild to moderately decreased |
| 30 - 44 | G3b | Moderate to severely decreased |
| 15 - 29 | G4 | Severely decreased (Pre-dialysis) |
| < 15 | G5 | Kidney failure (ESRD) |
References:
1. Lyman JL. B12 clearance and the measurement of GFR. Clin Chem. 1986.
[Clinical Chemistry]
2. KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013.
[KDIGO Official]
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