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Because mild and moderate kidney injury is poorly inferred from serum creatinine alone, NKDEP strongly encourages clinical laboratories to routinely estimate glomerular filtration rate (GFR) and report the value when serum creatinine is measured for patients 18 and older, when appropriate and feasible. An estimated GFR (eGFR) calculated from serum creatinine using the Modification of Diet in Renal Disease (MDRD) Study equation is a simple and effective way in which laboratories can help health care providers detect CKD among those with risk factors—diabetes, hypertension, cardiovascular disease, or family history of kidney disease. Providers also may use eGFR to monitor patients already diagnosed with CKD.
Use the MDRD Study Equation to Estimate GFR
There are several reasons for using the MDRD Study equation to estimate GFR, including:
- GFR is poorly inferred from serum creatinine alone. This is mainly because these are related inversely and nonlinearly to serum creatinine. The effects of age and gender, and to a lesser extent race, on creatinine production further cloud interpretation of a serum creatinine result.
- The normal serum creatinine reference interval does not necessarily reflect a normal GFR for a patient. Because the MDRD employs age, gender, and race, providers may observe that CKD is present despite a serum creatinine concentration that appears to fall within or just above the normal reference interval.
- The MDRD Study equation is the most thoroughly validated equation. The equation has been validated extensively in Caucasian and African American populations between the ages of 18 and 70* with impaired kidney function (eGFR <60 mL/min/1.73 m2) and has shown good performance for patients with all common causes of kidney disease1.
- The MDRD Study equation is currently superior to other methods of approximating GFR. Direct comparison of the MDRD to other equations such as Cockcroft-Gault1,2 and to creatinine clearance measured from 24-hour urine collections has demonstrated this superiority.3 Note that creatinine clearance should be considered when the patient's basal creatinine production is very abnormal. This may be the case with patients of extreme body size or muscle mass (e.g., obese, severely malnourished, amputees, paraplegics, or other muscle-wasting diseases), or with unusual dietary intake (e.g., vegetarian, creatine supplements).
- Measurement of kidney function (eGFR or creatinine clearance) is essential once albuminuria is discovered.
It is not always suitable to assess kidney function using eGFR. Learn more about situations where use of the MDRD Study equation is not recommended.
*The equation has not been validated in patients older than 70, but an MDRD-derived eGFR may still be a useful tool for providers caring for patients older than 70.
Use the Correct MDRD Study Equation
Laboratories should program their information systems to use the MDRD Study equation to automatically estimate and report GFR for patients ages 18 and older, when appropriate and feasible. It is critical, however, that laboratories use the correct MDRD equation, based on the calibration of its creatinine measurement. Learn more about which equation to use.
Original MDRD Study equation (for creatinine methods NOT calibrated to an IDMS reference method)
GFR (mL/min/1.73 m2) = 186 x (Scr)-1.154 x (Age)-0.203 x (0.742 if female) x (1.212 if African American)
IDMS-traceable MDRD Study equation (for creatinine methods calibrated to an IDMS reference method)
GFR (mL/min/1.73 m2) = 175 x (Scr)-1.154 x (Age)-0.203 x (0.742 if female) x (1.212 if African American)
The equation does not require weight or height variables because the results are reported normalized to 1.73 m2 body surface area, which is an accepted average adult surface area.
Reduce Rounding Errors
NKDEP recommends using serum creatinine values in mg/dL to two decimal places (e.g., 0.95 mg/dL) OR values in µmol/L to the nearest whole number (e.g., 84 µmol/L) when calculating eGFR using the MDRD Study equation. This practice will reduce rounding errors that may contribute to imprecision in the eGFR value.
References
1. Poggio ED, Wang X, Greene T, Van Lente F, Hall PM. Performance of the modification of diet in renal disease and Cockcroft-Gault equations in the estimation of GFR in health and in chronic kidney disease. Journal of the American Society of Nephrology. 2005;16(2):459–466.
2. Coresh J, Stevens LA. Kidney function estimating equations: where do we stand? Current Opinion in Nephrology and Hypertension. 2006;15(3):276–284.
3. Walser M. Assessing renal function from creatinine measurements in adults with chronic renal failure. American Journal of Kidney Disease. 1998;32(1):23–31.
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