Recommended Method for Calculating eGFR
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 Approach
The 2021 CKD-EPI equation represents the current gold standard for eGFR calculation, developed through a comprehensive reassessment by the National Kidney Foundation and American Society of Nephrology Task Force. 1 This equation:
- Eliminates race as a variable while maintaining accuracy across diverse populations 1
- Provides more accurate estimates than the MDRD equation, especially at GFR >60 mL/min/1.73 m² 2, 3
- Uses only standardized serum creatinine, patient age, and sex 2, 4
- Should be automatically reported by clinical laboratories whenever serum creatinine is ordered 2, 4
Laboratory Requirements for Accuracy
For valid eGFR calculations, laboratories must meet specific standards:
- Serum creatinine must be measured using assays calibrated to isotope-dilution mass spectrometry (IDMS) 2, 4
- Creatinine values <1 mg/dL should be reported to two decimal places 2, 4
- Assays must be traceable to international reference standards 4
When to Add Cystatin C
Use the combined CKD-EPI creatinine-cystatin C equation (eGFRcr-cys) for confirmation in specific clinical situations: 1, 2
- eGFR 45-59 mL/min/1.73 m² without other markers of kidney damage 1, 4
- Extremes of muscle mass or body composition 1, 2, 4
- Advanced cirrhosis 1, 2
- Cancer with high cell turnover 1, 2
- When eGFRcr is thought to be inaccurate 1, 4
The combined creatinine-cystatin C equation yields more accurate GFR estimates overall, particularly among persons with large discordances between eGFRcr and eGFRcys. 1
Equations to Avoid or Use with Caution
The Cockcroft-Gault equation should not be used as a primary method because it overestimates kidney function in CKD stages 3-5 and is less accurate than CKD-EPI. 2, 5
The MDRD equation has been superseded by CKD-EPI 2021 and should no longer be used for routine clinical practice, as it systematically underestimates GFR at higher values and is not valid for eGFR >60 mL/min/1.73 m². 2, 3
When to Measure GFR Directly
Consider measured GFR using exogenous markers (iohexol or radioisotopic markers) when: 2, 4
- Precise GFR measurement is critical for clinical decision-making 2, 4
- Calculating doses of potentially toxic drugs cleared by kidneys 2
- Extremes of age or body size 2
- Even combined eGFRcr-cys may be inaccurate (e.g., radical differences in body composition, high rates of inflammation and catabolism) 1
Special Populations
For children, use pediatric-specific equations such as the Schwartz equation or Full Age Spectrum (FAS) equation, not adult equations. 2 Laboratories measuring creatinine in children should use enzymatic assays rather than Jaffe methods due to higher relative contribution of non-creatinine chromogens. 1, 5
Implementation Strategy
The Task Force emphasized that the same equation should be used consistently within geographical regions to avoid confusion and variation in eGFR values across clinical settings. 1, 4 Clinical providers should request their laboratory to automatically report eGFR using the 2021 CKD-EPI equation whenever serum creatinine is ordered, rather than relying on serum creatinine values alone. 2, 4