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.
Indirect markers
Most doctors use the plasma concentrations of creatinine, urea, and electrolytes to determine renal function. These measures are adequate to determine whether a patient is suffering from kidney disease.Unfortunately, BUN and creatinine will not be outside the normal range until 60% of total kidney function is lost. Hence, creatinine clearance is a more accurate measure and is used whenever renal disease is suspected or careful dosing of nephrotoxic drugs is required.
Glomerular filtration rate
In 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. But, if the GFR has significantly declined from a previous test result, this can be an early indicator of kidney disease requiring medical intervention. The sooner kidney dysfunction is diagnosed and treated, the greater odds of preserving remaining nephrons, and preventing the need for dialysis.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.
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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, as originally published, is:
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A modification of this formula, useful for the common units of measure, is:
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It is named after the scientists who first published the formula (Cockroft & Gault, 1976). The equation is popular because it is easy to calculate.
MDRD formula
The most recently advocated 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
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Creatinine levels in µmol/L can be converted to mg/dL 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.
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References
- Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41. PMID 1244564
- 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.
See also
- Clearance
- Creatinine clearance
- Kt/V
- Pharmacokinetics
- Renal clearance ratio
- Standardized Kt/V
- Urea reduction ratio
External links
- [Online calculator] for Cockroft-Gault and MDRD formulae.
- [Schwartz formula] - cornell.edu
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