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CDHB

Renal Function - Assessment

Glomerular filtration rate (GFR) is the best measure of renal function. The plasma creatinine concentration alone is not a sufficiently accurate predictor of glomerular filtration rate, particularly for small or elderly patients.

• In practice GFR is estimated either by the Cockroft and Gault or eGFR formulae. Both provide a guide to GFR which is adequate for most clinical situations. These formulae are unreliable at extremes of weight and/or when the creatinine is changing and for a patient with a near normal GFR although the new CKD-EPI equation performs better in this regard.
• Note that GFR falls by approximately 0.75 - 1 mL/min per year over the age of 40.
• If renal function is impaired:
  • Modify the dose of drugs that are renally excreted.
  • Try to use drugs that are not nephrotoxic.
  • If a critical dose drug known to be nephrotoxic must be used, consider getting a more accurate guide to GFR such as a radionuclide GFR measurement.

Cockcroft and Gault Formula

Cockcroft and Gault (Nephron 1976, 16:31-41) developed a simple bedside formula to predict the creatinine clearance without having to collect urine and using the variables of plasma creatinine concentration, body weight, sex and age. The modified formula is as follows:

• Ideal body weight (males) = 50 kg + 0.9 kg for each cm over 150 cm in height.
• Ideal body weight (females) = 45 kg + 0.9 kg for each cm over 150 cm in height.

  Note: Use actual body weight if this is less than the ideal body weight.

This formula has not been tested in infants or young children, but is accurate from the age of 12 years onwards.

CKD-EPI

This formula is used by Canterbury Health Laboratories to estimate GFR from measured serum creatinine. It takes into account serum creatinine, age and gender. It has now superseded the MDRD formula for estimating GFR based on serum creatinine at Christchurch Hospital. The prime reason for its adoption was that at near normal GFR, i.e., >60 mL/min, the CKD-EPI formula has less bias and greater accuracy.

Note: All estimates of GFR from serum creatinine in patients with GFR >60 mL/min are still relatively approximate.

Topic Code: 2740