How to Calculate eGFR
Clinical laboratories should automatically calculate and report eGFR using the 2021 CKD-EPI creatinine equation (race-free version) whenever serum creatinine is measured, as this is the current standard recommended by KDIGO guidelines. 1
Primary Calculation Method
Use the 2021 CKD-EPI creatinine equation as the preferred formula for estimating GFR in adults, as it performs better than older equations (MDRD, Cockcroft-Gault) especially at higher GFR levels and eliminates race-based adjustments. 1
What You Need to Calculate eGFR:
- Serum creatinine (measured using standardized assay calibrated to IDMS reference methodology)
- Age (in years)
- Sex (male or female)
- No additional testing required - these variables are automatically incorporated into the equation 2
Alternative Equations (When CKD-EPI 2021 is Not Available):
For routine clinical use, the abbreviated MDRD (aMDRD) formula remains acceptable: 3
eGFR (mL/min/1.73 m²) = 186 × [serum creatinine (mg/dL)]^-1.154 × [age (years)]^-0.203 × [0.742 if female] × [1.21 if African American]
The Cockcroft-Gault equation can be used but requires body weight and produces unadjusted creatinine clearance (not standardized to BSA): 1
CrCl = [(140 - age) × weight (kg) × 0.85 if female] / [72 × serum creatinine (mg/dL)]
How eGFR is Reported
- Reported as a whole number rounded to the nearest integer 3, 2
- Standardized to body surface area of 1.73 m² using units of mL/min per 1.73 m² 3, 2
- Values <60 mL/min/1.73 m² should be flagged as "decreased" to indicate potential chronic kidney disease 3, 2
- Laboratories automatically report eGFR alongside serum creatinine without requiring a separate order 2, 1
When eGFR May Be Inaccurate
Consider confirmatory testing with cystatin C or measured GFR in these situations where creatinine-based eGFR is less reliable: 3, 2
- Extremes of muscle mass (bodybuilders, amputees, severe malnutrition, muscle-wasting diseases) 3, 2
- Extremes of body size (severe obesity or cachexia) 3, 1
- Very elderly patients (>80 years) where age-related muscle loss affects creatinine 3
- Vegetarian diet (lower creatinine production) 1
- Rapidly changing kidney function (acute kidney injury) 1
- When dosing potentially toxic drugs that require precise GFR assessment 1
Adjusting eGFR for Medication Dosing
For drug dosing in patients with extreme body sizes, adjust the standardized eGFR to actual body surface area: 2
Adjusted eGFR = eGFR × (patient's BSA / 1.73)
For routine CKD screening and staging, use the standard eGFR (normalized to 1.73 m²) without adjustment. 2
Clinical Interpretation
- eGFR ≥60 mL/min/1.73 m² = Normal or mildly decreased kidney function 3, 2
- eGFR 45-59 mL/min/1.73 m² = Stage 3a CKD (mild to moderate reduction) 3
- eGFR 30-44 mL/min/1.73 m² = Stage 3b CKD (moderate to severe reduction) 3
- eGFR <30 mL/min/1.73 m² = Stages 4-5 CKD (severe reduction; refer to nephrology) 3
Important Caveats
Confirm abnormal eGFR with repeat testing - biological variability in creatinine can be >20%, so two of three measurements over 3-6 months should be abnormal before diagnosing CKD. 3
Transient elevations in creatinine can occur with exercise within 24 hours, infection, fever, marked hyperglycemia, or marked hypertension, independent of true kidney damage. 3
Laboratory standardization is critical - serum creatinine must be measured using assays calibrated to international standards (IDMS) with coefficient of variation <2.3% and bias <3.7% for accurate eGFR calculation. 2