Dr Graham Jones
Department of Chemical Pathology

GFR Estimation (MDRD equation)

The Glomerular Filtration Rate (GFR) is the best overall measure of renal function. The most accurate measurement of GFR requires complex technology which is not routinely available. Alternate estimates of the GFR (eGFR) can be made by the following methods:

• Estimate of GFR (eGFR) by MDRD formula (which requires age, sex and serum creatinine only). The MDRD formula for estimation of GFR is recommended for routine use and is reported with all requests for serum creatinine. See below for further information.

• Estimate of GFR by the Cockcroft and Gault equation (which requires patient age, sex, weight or height, and serum creatinine). This estimate is recommended for drug dosing decisions (on-line calculator).

• Measurement of Creatinine Clearance (which requires a serum creatinine measurement and a 24 hour urine collection). This is the least reliable method for routine use but is valuable in extremes of body composition where the estimation formulae above are not reliable.

eGFR using MDRD Formula      (on line calculator)

eGFR (mL/min/1.73m²)= 175 x [SerumCreatinine(umol/L) x 0.0113]^-1.154 x Age(years)^-0.203 (x 0.742 if female)

Note: This version of the MDRD formula (with 175 in plce of 186 as the first coefficient), is sutiable for use with creatinine assays aligned with the international reference method.

eGFR by MDRD: NOTES

• eGFR using the MDRD equation will be reported with all requests for serum creatinine.
• This is a recommendation of the peak renal and pathology bodies in Australia (Chronic Kidney Disease and Automatic reporting of glomerular filtration rate: a position statement. MJA 2005;183 (3): 138-141. www.mja.com.au)
• The formula is named after the Modification of Diet in Renal Disease study in the USA.
• The results are expressed relative to a standard body surface area of 1.73 m² to allow for different body sizes. The units are mL/min/1.73m².
• The equation is only valid in persons 18 years of age or older and not reported in younger patients.
• Results >60 mL/min/1.73m² are likely to deviate from the true value and are not routinely reported.
• The use of the eGFR in patients on dialysis is inappropriate and will give misleading results.
• The formula has not been validated for drug dosing and alternate estimates of GFR such as the Cockcroft and Gault methods should be used.
• The formula has not been validated in Aboriginals, Pacific Islanders or Chinese people and results may deviate from the true value. For African-American patients multiply result by 1.21
• Results may deviate from true values in patients with exceptional dietary intake (eg vegetarian diet, high protein diet, creatine supplements); extremes of body composition (eg lean, obese, paraplegia), or severe liver disease.
• Drug Dosing information in Australia (eg MIMS, Australian Medicines Handbook, Therapeutic Guidelines) is based on Cockcroft and Gault estimates of creatinine clearance. Cockcroft and Gault is recommended for drug dosing decisions.

eGFR by MDRD: INTERPRETATION

The MDRD estimate of GFR can be used for identification of moderate to severe decrease in renal function and as a staging tool for Chronic Kidney Disease (CKD)

• eGFR >59 mL/min/1.73m² - Normal GFR or mild decrease in GFR
• eGFR  30 - 59 mL/min/1.73m² - Moderate decrease in GFR (Stage 3 CKD)
• eGFR  15 - 29 mL/min/1.73m² - Severe decrease in GFR (Stage 4 CKD)
• eGFR  <16 mL/min/1.73m² - End-stage kidney failure (Stage 5 CKD)

Note that:

• The MDRD eGFR is NOT a sensitive test for renal failure and cannot be used to detect mild renal impairment (GFR 60 - 90 mL/min/1.73m²).
• Results >60 mL/min/1.73m² do not necessarily indicate normal renal function.
• The most sensitive routine test for small reductions in GFR is a comparison of a serum creatinine concentration with a previous result from the same patient. An increase of 15% or more in serum creatinine indicates a significant fall in GFR.

For futher information and clinical action plans see the Kidney Health Australia website (www.kidney.org.au/calculator)