How to Calculate eGFR
Use the 2021 CKD-EPI creatinine equation without race as the standard method for calculating eGFR in adults, which requires only serum creatinine, age, and sex. 1
Primary Calculation Method: CKD-EPI 2021 Equation
The CKD-EPI 2021 equation is the recommended standard for routine clinical practice and should automatically be reported by laboratories whenever serum creatinine is ordered. 2, 1 This equation:
- Uses standardized serum creatinine (calibrated to IDMS), patient age, and sex—no race variable 1
- Provides more accurate estimates than MDRD, especially at GFR >60 mL/min/1.73 m² 3, 1
- Reports values rounded to the nearest whole number in units of mL/min per 1.73 m² 2
- Flags any eGFR <60 mL/min per 1.73 m² as low 2
Critical technical requirement: Serum creatinine must be measured using assays calibrated to isotope-dilution mass spectrometry (IDMS) with coefficient of variation <2.3% and bias <3.7%, with values <1 mg/dL reported to two decimal places. 2, 1
Alternative Equations for Specific Clinical Situations
When to Add Cystatin C
Add cystatin C measurement and use the combined CKD-EPI creatinine-cystatin C equation when: 1
- eGFR 45-59 mL/min/1.73 m² without other markers of kidney damage (to confirm CKD diagnosis) 3, 1
- Extremes of muscle mass are present 1
- Advanced cirrhosis or cancer 1
- Uncertainty about accuracy of creatinine-based eGFR 2
The combined equation provides confirmatory assessment, as two-thirds of patients with eGFRcreat <60 mL/min/1.73 m² will have CKD confirmed by eGFRcys <60 mL/min/1.73 m². 3
Older Equations (Generally Not Recommended)
MDRD equation (1999): 3
- Formula: 175 × (serum creatinine)^-1.154 × (age)^-0.203 × (0.742 if female) × (1.212 if African American) 3
- More accurate than Cockcroft-Gault for GFR <90 mL/min/1.73 m² but not valid for eGFR >60 mL/min/1.73 m² 1
- Has been superseded by CKD-EPI 2021 1
Cockcroft-Gault equation (1976): 3
- Formula: [(140 - age) × weight (kg) × (0.85 if female)] / [72 × serum creatinine (mg/dL)] 3
- Overestimates kidney function in CKD stages 3-5 and is less accurate than CKD-EPI 1
- May be preferable in subjects younger than 65 years compared to MDRD 3
Special Populations
Pediatric patients: Use pediatric-specific equations such as the Schwartz equation or Full Age Spectrum (FAS) equation, not adult formulas. 1 For children, a simplified formula is: eGFR (mL/min) = 0.55 × length (cm) / serum creatinine (mg/dL). 3
Glomerular disease patients: No eGFR equation has been specifically validated; consider measured GFR using exogenous markers for critical decisions, as hypoalbuminemia, glucocorticoids, and low muscle mass may affect estimates. 1
When eGFR is Inaccurate: Measured GFR
Consider measured GFR (using iohexol or radioisotopic markers like iothalamate) when: 2, 1
- Precise GFR measurement is critical for clinical decision-making 1
- Extremes of age or body size 1
- Calculating doses of potentially toxic drugs excreted by kidneys 1
- Acute kidney injury 1
- Very elderly patients or extremes of muscle mass 2
Measured GFR removes the effect of endogenous non-GFR determinants (muscle mass variability) that limit accuracy of creatinine-based estimates at the individual patient level. 3
Common Pitfalls to Avoid
Do not use serum creatinine alone to assess kidney function—it significantly overestimates renal clearance, particularly in elderly patients. 3, 2, 1
Be aware that creatinine-based eGFR has significant non-GFR variability from muscle mass that is not adequately accounted for in equations, limiting personalized care. 3
Exercise within 24 hours, infection, fever, congestive heart failure, marked hyperglycemia, menstruation, and marked hypertension may elevate measurements independently of kidney damage. 3
In diabetic patients with poor glycemic control, eGFR may overestimate renal function and be relatively inaccurate compared to non-diabetic patients. 4