Renal function

Associate Editor-In-Chief:

Overview
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.


 * $${GFR corrected} = \frac{{GFR} \times {1.73}} {BSA}$$

BSA can be calculated on the basis of weight and height.

Cockcroft-Gault formula
A commonly used surrogate marker for actual creatinine clearance is the Cockcroft-Gault formula, which employs creatinine measurements and a patient's weight to predict the clearance. The formula, as originally published, is:


 * $${x} = \frac{(140-{age}) \times {ideal body weight} \times {0.85 (if female)}} {{72} \times {creatinine}}$$

This formula expects weight (actually mass) to be measured in kilograms and creatinine to be measured in mg/dL, as is standard in the USA. The resulting value is multiplied by a constant of 0.85 if the patient is female. This formula is useful because the calculations are relatively simple and can often be performed without the aid of a calculator.

A modification of this formula, useful for the common units of measure, is:


 * $${x} = \frac{(140-{age}) \times {weight} \times {constant} } {creatinine}$$

This formula uses metric units (weight in kilograms, creatinine in µmol/L). The constant is 1.23 for men and 1.04 for women.

It is named after the scientists who first published the formula (Cockcroft & 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


 * $${x} = 186 \times {creatinine}^{-1.154} \times {age}^{-0.203} \times {constant}$$

For creatinine in µmol/L


 * $${x} = 32788 \times {creatinine}^{-1.154} \times {age}^{-0.203} \times {constant}$$

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 in µmol/L can be converted to mg/dL by dividing them by 88.4. The 32788 number above is equal to 186×88.41.154.

A more elaborate version of the MDRD equation also includes serum albumin and blood urea nitrogen levels:


 * $${GFR} = 170 \times {[Pcr]}^{-0.999} \times {[Age]}^{-0.176} \times {[0.762\ if\ patient\ is\ female]} \times {[1.180\ if\ patient\ is\ black]} \times {[SUN]}^{-0.170} \times {[Alb]}^{+0.318}$$


 * $${[Pcr]} = serum\ creatinine\ concentration\ (mg/dL)$$
 * $${[SUN]} = serum\ urea\ nitrogen\ concentration\ (mg/dL)$$
 * $${[Alb]} = serum\ albumin\ concentration\ (g/dL)$$

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.


 * $${x} = \frac{ {k} \times {height} }{creatinine}$$

k - for height in some units and creatine in some other units, the constant is 0.33-0.45 in infants, 0.55 in children or adolescent girls, or 0.70 in adolescent boys.