Renal function
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In medicine (nephrology) renal function is an indication of the state of the kidney and its role in physiology.
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Indirect markers
Most doctors use creatinine, urea and electrolytes to determine renal function. These measures are adequate to determine whether a patient is suffering from kidney disease.
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.
- <math>{GFR corrected} = \frac{ GFR \times 1.73 } {BSA}<math>
BSA can be calculated on the basis of weight and height.
Cockroft-Gault formula
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:
- <math>{x} = \frac{(140-{age}) \times {weight} \times {constant} } {creatinine}<math>
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).
MDRD formula
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
- <math>{x} = 1.86 \times {creatinine}^{-1.154} \times {age}^{-0.203} \times {constant}<math>
For creatinine in µmol/L
- <math>{x} = 0.021 \times {creatinine}^{-1.154} \times {age}^{-0.203} \times {constant}<math>
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.
A more elaborate version of the MDRD equation also includes albumin and blood urea nitrogen levels.
Children: Schwartz
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[1] (http://www-users.med.cornell.edu/~spon/picu/calc/crclschw.htm) warns that the result may overstate the GFR.
- <math>{x} = \frac{ {k} \times {height} }{creatinine}<math>
k - the constant is 0.33-0.45 in infants, 0.55 in children or adolescent girls, or 0.70 in adolescent boys.
References
- 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.
External links
- Online calculator (http://www.nephron.com/) for Cockroft-Gault and MDRD formulae.