Calculation of Estimated Glomerular Filtration Rate (eGFR)
The CKD-EPI creatinine equation is currently recommended as the preferred method for estimating glomerular filtration rate (eGFR) in routine clinical practice, with combined creatinine-cystatin C equations providing improved accuracy in specific clinical scenarios. 1
Primary eGFR Calculation Methods
CKD-EPI Creatinine Equation (Preferred)
The 2021 race-free CKD-EPI creatinine equation is currently the most widely recommended equation for initial evaluation of kidney function 1. This equation:
- Provides greater precision than other equations, especially for GFR >60 ml/min/1.73m²
- No longer includes race as a variable (previous versions did)
- Requires only serum creatinine, age, and sex
- Is valid across a wide range of GFR values
MDRD (Modification of Diet in Renal Disease) Equation
- More precise for GFR <90 ml/min/1.73m² 1
- Less accurate at higher GFR values (systematically underestimates)
- Now considered less preferred than CKD-EPI for general use
Cockcroft-Gault Equation
- Formula: [(140-age) × weight (kg) × 0.85 if female] / [serum creatinine (mg/dl) × 72]
- Historically used for medication dosing
- Less precise in elderly patients and those with extreme muscle mass values
- Estimates creatinine clearance rather than GFR
Enhanced Accuracy Methods
Combined Creatinine-Cystatin C Equations
- When to use: When creatinine-based estimates are suspected to be inaccurate 1, 2
- Provides improved accuracy in patients with:
- Abnormal muscle mass
- Malnutrition
- Extreme body sizes
- Vegetarian diet
- Paraplegia/quadriplegia
Direct GFR Measurement
For maximum precision in critical decisions (gold standard methods):
- Inulin clearance (historical gold standard)
- Iohexol clearance
- Radioisotopic methods (51Cr-EDTA, 99mTc-DTPA)
- 24-hour urine creatinine clearance (less precise but more accessible)
Clinical Considerations
Limitations of eGFR Calculations
- All equations have reduced accuracy in:
- Extremes of age or body size
- Severe malnutrition or obesity
- Rapidly changing kidney function
- Acute kidney injury (steady state is required)
- Pregnancy
Special Populations
- Children: Use specialized pediatric equations (modified Schwartz or Full Age Spectrum formulas) 1
- Elderly: CKD-EPI generally performs better than other equations 3
- Hospitalized patients: Cystatin C-based equations may be more accurate in malnourished patients 4
Laboratory Considerations
- Standardization of creatinine assays is critical for accuracy
- Enzymatic creatinine assays are preferred in children due to fewer interfering substances 1
- Laboratories should implement quality control processes that include the expected range of values for the population being tested
Clinical Application
- An eGFR <60 ml/min/1.73m² for ≥3 months indicates chronic kidney disease
- Minor elevations in serum creatinine may correspond to substantial reductions in GFR
- eGFR should be interpreted in the clinical context, considering the patient's overall condition and the presence of other markers of kidney damage
By following these guidelines for eGFR calculation, clinicians can more accurately assess kidney function, leading to improved diagnosis, risk stratification, and management of patients with kidney disease.