Calculate estimated GFR (eGFR) from serum creatinine levels to assess kidney function.
Use of any serum creatinine-based estimate requires that kidney function be at a steady state. eGFR should be used with caution in acutely ill or hospitalized patients who may exhibit rapidly changing kidney function.
Adults: Use either the Modification of Diet in Renal Disease (MDRD) Study equation or the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation.
Patients under the age of 18: Calculate eGFR using the Schwartz equation for patients under age 18.
Caution: It is important to know the method used to measure creatinine in a blood, serum, or plasma sample, as it will affect the formula for estimating GFR in children. Determine which calculator to use.
Reasons for Using the MDRD Study or CKD-EPI Equation to Estimate GFR
Because mild and moderate kidney injury is poorly inferred from serum creatinine alone, NKDEP strongly recommends the use of either the MDRD Study or CKD-EPI equation to estimate GFR from serum creatinine in adults. NKDEP also encourages clinical laboratories to routinely estimate GFR and report the value when serum creatinine is measured for patients 18 and older, when appropriate and feasible.
- GFR is poorly inferred from serum creatinine alone. GFR is related inversely and nonlinearly to serum creatinine. Age, gender, and race all affect muscle mass and, in turn, serum creatinine. Both the MDRD Study and CKD-EPI equations include variables to account for serum creatinine variation with age, gender, and race.
- The normal serum creatinine reference range does not necessarily reflect a normal GFR for a patient. Because the MDRD and CKD-EPI equations employ 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 range.
- The MDRD Study and CKD-EPI equations are the most widely used and thoroughly validated equations. Both equations have been validated extensively in Caucasian and African American populations with impaired kidney function (eGFR < 60 mL/min/1.73 m2) and have shown good performance for patients with all common causes of kidney disease. Additionally, the CKD-EPI equation has demonstrated improved accuracy in populations with eGFR levels >60 mL/min/1.73 m2 compared to the MDRD Study equation; however, the influence of imprecision of creatinine assays on the uncertainty of an eGFR value is greater at higher eGFR values and should be considered when assessing eGFRs > 60 mL/min/1.73 m2.
- The MDRD Study and CKD-EPI equations are superior to traditional methods of approximating GFR. Direct comparison of the MDRD and CKD-EPI equations to the Cockcroft-Gault equation and to creatinine clearance measured from 24-hour urine collections has demonstrated their improved accuracy.
Assessing GFRs above 60 mL/min/1.73 m2
A laboratory that reports eGFR numeric values > 60 mL/min/1.73 m2 should use the CKD-EPI equation, because the CKD-EPI equation is more accurate for values > 60 mL/min/1.73 m2 than is the MDRD Study equation. However, the influence of imprecision of creatinine assays on the uncertainty of an eGFR value is greater at higher eGFR values and should be considered when assessing eGFR values > 60 mL/min/1.73 m2.
Estimated glomerular filtration rate (eGFR) calculated using either the MDRD Study equation or the CKD-EPI equation is an estimate of GFR, not the actual GFR. Both equations were derived from large population studies and will generate an estimate of the mean GFR in a population of patients with the same age, gender, race, and serum creatinine. However, the actual GFRs of those individuals will be distributed around that eGFR. An analogous estimate would be the estimated date of confinement for a pregnant woman based on her last menstrual period. This is the best estimate of the delivery date but, in fact, only a small minority of women actually deliver on that date.
When Not to Use the Creatinine-based Estimating Equations: Although the best available tool for estimating kidney function, eGFR derived from the MDRD Study or CKD-EPI equations may not be suitable for all populations. All creatinine-based estimates of kidney function are only useful when renal function is stable. Serum creatinine values obtained while kidney function is changing will not provide accurate estimates of kidney function.
Additionally, the equations are not recommended for use with:
- Patients under the age of 18. The Bedside Schwartz equation should be used to estimate GFR for patients under age 18. Caution: It is important to know the method used to measure creatinine in a blood, serum, or plasma sample, as it will affect the formula for estimating GFR in children. Determine which calculator to use.
- Patients with unstable creatinine concentrations. This includes pregnant women; patients with serious comorbid conditions; and hospitalized patients, particularly those with acute kidney injury.
- Patients with extremes in body size or muscle mass, or with altered dietary intakes. This includes, but is not limited to, individuals who are amputees, paraplegics, bodybuilders, or obese; patients who have a muscle-wasting disease or a neuromuscular disorder; and those suffering from malnutrition, eating a vegetarian or low-meat diet, or taking creatine dietary supplements.
Application of either the MDRD Study or CKD-EPI equation to these patient groups may lead to errors in GFR estimation. GFR-estimating equations have poorer agreement with measured GFR for ill hospitalized patients and for people with near normal kidney function than for the patients in the MDRD or CKD-EPI Study.