How eGFR is Calculated
Use the 2021 CKD-EPI creatinine equation without race as the standard method for estimating GFR in adults, which requires only serum creatinine, age, and sex. 1, 2
Primary Calculation Method
The CKD-EPI 2021 equation is the current gold standard for eGFR calculation in routine clinical practice, replacing older equations that included race-based adjustments. 1, 2 This equation:
- Uses standardized serum creatinine (calibrated to IDMS reference methodology) 2
- Incorporates patient age and sex 2
- Does not use race as a variable (unlike the 2009 version) 1
- Provides more accurate estimates than MDRD, especially at GFR >60 mL/min/1.73 m² 3
Critical laboratory requirement: Serum creatinine must be measured using assays calibrated to isotope-dilution mass spectrometry (IDMS) and traceable to international reference standards, with values <1 mg/dL reported to two decimal places. 2
Alternative Equations for Specific Situations
When CKD-EPI May Be Inaccurate
Add cystatin C measurement and use the combined CKD-EPI creatinine-cystatin C equation when: 1, 2
- Extremes of muscle mass (very high or very low)
- Severe obesity or malnutrition
- Advanced cirrhosis
- Cancer
- Vegetarian diet
- Paraplegia or quadriplegia
- eGFR 45-59 mL/min/1.73 m² without other markers of kidney damage (for confirmation) 2
Older Equations (Historical Context)
Cockcroft-Gault equation (1976): 4
- Formula: (140 - age) × weight (kg) × 0.85 (if female) / [serum creatinine (mg/dL) × 72]
- Requires body weight
- May overestimate kidney function in CKD stages 3-5 1
- Less accurate than CKD-EPI 4
MDRD equation (1999): 4
- More accurate than Cockcroft-Gault for GFR <90 mL/min/1.73 m² 4
- Not valid for eGFR >60 mL/min/1.73 m² 4
- Has been superseded by CKD-EPI 2021 1
Special Populations
Children
Use pediatric-specific equations: 4
- Schwartz equation and its modifications
- Full Age Spectrum (FAS) equation (valid across all ages including children) 4
Glomerular Disease Patients
Important caveat: No eGFR equation has been specifically validated for glomerular diseases or nephrotic syndrome. 4 In these patients:
- Hypoalbuminemia may cause overestimation of true GFR due to increased tubular creatinine secretion 4
- Glucocorticoids may increase serum cystatin C, potentially underestimating eGFR 4
- Low muscle mass overestimates eGFR using creatinine-based formulas 4
- Consider measured GFR using exogenous markers (iohexol, radioisotopic clearance) for critical decisions 4, 2
When to Measure GFR Directly
Measured GFR (using iohexol or radioisotopic markers) should be considered when: 4, 2
- Precise GFR measurement is critical for clinical decision-making
- Extremes of age or body size
- Rapidly changing kidney function
- Calculating doses of potentially toxic drugs excreted by kidneys 4
- Acute kidney injury (all estimates are invalid in non-steady-state conditions) 4
Implementation in Practice
Request your laboratory to automatically report eGFR using CKD-EPI 2021 whenever serum creatinine is ordered. 2 The equation should be applied consistently within your geographical region. 2
Do not use serum creatinine alone to assess kidney function—always calculate or obtain eGFR. 4 Minor elevations in creatinine can represent substantial GFR reductions, particularly in elderly patients, women, and those with low muscle mass. 4