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Glomerular filtration rate
In known renal patients, the glomerular filtration rate (GFR) is used. This is calculated by comparing urine creatinine levels with the blood test results. It gives a more precise indication of the state of the kidneys. The GFR is expressed in ml/min. For most patients, a GFR over 60 ml/min is adequate. GFR measurements can aid a nephrologist in deciding when to initiate dialysis or renal transplantation.
Very often, the GFR is expressed as ml/min/1.73 m2. This is an indication that the GFR needs to be corrected for the body surface area (BSA). While most adults have a BSA that approaches 1.7 (1.6-1.9), extremely obese or slim patients should have their GFR corrected for their actual BSA.
BSA can be calculated on the basis of weight and height.
A commonly used surrogate marker for actual creatinine clearance is the Cockroft-Gault formula, which employs creatinine measurements and a patient's weight to predict the clearance.
The formula is:
This formula uses metric units (weight in kilograms, creatinine in µmol/L). The constant is 1 for men and 0.85 for women.
It is named after the scientists who first published the formula (Cockroft & Gault, 1976).
Another commonly-used formula for calculating the GFR is the one that was developed as a result of the Modification of Diet in Renal Disease (MDRD) study (Levey et al 1999).
For creatinine in mg/dL
For creatinine in µmol/L
The constant is 1 for a white male, and is multiplied with 0.742 for females and multiplied with 1.21 for African Americans.
Creatinine levels is in mg/dL can be converted to µmol/L by dividing them by 88.4.
In children, the Schwartz formula is used. This employs the serum creatinine, the child's height and a constant to estimate the creatinine clearance. An online calculator warns that the result may overstate the GFR.
k - the constant is 0.33-0.45 in infants, 0.55 in children or adolescent girls, or 0.70 in adolescent boys.
- Cockroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron 1976;16:31-42.
- Levey AS, Bosch JP, Lewis JB, Greene T, Rogers N, Roth D. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999;130:461-470. PMID 10075613.
- Online calculator for Cockroft-Gault and MDRD formulae.
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