# Renal function

In medicine (nephrology) renal function is an indication of the state of the kidney and its role in physiology.

 Contents

## 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.

[itex]{GFR corrected} = \frac{ GFR \times 1.73 } {BSA}[itex]

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:

[itex]{x} = \frac{(140-{age}) \times {weight} \times {constant} } {creatinine}[itex]

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

[itex]{x} = 1.86 \times {creatinine}^{-1.154} \times {age}^{-0.203} \times {constant}[itex]

For creatinine in µmol/L

[itex]{x} = 0.021 \times {creatinine}^{-1.154} \times {age}^{-0.203} \times {constant}[itex]

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 (http://www-users.med.cornell.edu/~spon/picu/calc/crclschw.htm) warns that the result may overstate the GFR.

[itex]{x} = \frac{ {k} \times {height} }{creatinine}[itex]

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.

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