eGFR Calculation in Clinical Practice
Current Standard Method
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
The 2021 CKD-EPI equation represents the most significant update to GFR estimation, eliminating race as a variable while maintaining accuracy across diverse populations. 2 This equation supersedes older formulas including the MDRD and original CKD-EPI equations that included race coefficients. 1
Required Variables for Standard Calculation:
- Serum creatinine (must be IDMS-standardized, reported to two decimal places if <1 mg/dL) 1
- Age (in years) 2
- Sex (male or female) 2
The equation does NOT require body weight, height, or race. 1, 3
When to Add Cystatin C Measurement
Add cystatin C and use the combined CKD-EPI creatinine-cystatin C equation when:
- Extremes of muscle mass are present 1
- Advanced cirrhosis 1
- Cancer 1
- eGFR 45-59 mL/min/1.73 m² without other markers of kidney damage (confirmatory testing) 1
The combined creatinine-cystatin C equation provides the most accurate GFR estimates overall and reduces bias from non-GFR determinants of creatinine. 4
Older Equations: When They May Still Apply
MDRD Equation (1999)
- More accurate than Cockcroft-Gault for GFR <90 mL/min/1.73 m² 1
- Not valid for eGFR >60 mL/min/1.73 m² (systematically underestimates at higher values) 2, 3, 5
- Has been superseded by CKD-EPI 2021 for routine use 1
- Variables: serum creatinine, age, sex, race 2
Cockcroft-Gault Equation (1976)
- May overestimate kidney function in CKD stages 3-5 1, 3, 4
- Less accurate than CKD-EPI across all GFR ranges 1
- Requires body weight, making it susceptible to errors in obese or edematous patients 2
- Variables: serum creatinine, age, sex, weight 2
Critical Pitfalls to Avoid
Never Use Serum Creatinine Alone
Serum creatinine concentration should never be used as the sole means to assess kidney function. 2, 3
Creatinine is affected by multiple non-GFR factors including:
- Muscle mass (elderly patients may have "normal" creatinine despite 50% GFR reduction) 2, 3
- Creatinine secretion and generation 2, 3
- Extrarenal excretion 2
- Diet (vegetarian diet) 2
Laboratory Standardization Requirements
- Creatinine assays must be calibrated to isotope-dilution mass spectrometry (IDMS) 1
- Enzymatic methods are more specific than Jaffé methods 3
- Laboratories should automatically report eGFR whenever serum creatinine is ordered 1
When to Measure GFR Directly
Consider measured GFR using exogenous markers (iohexol or radioisotopic tracers) when:
- Extremes of age or body size 2, 1
- Severe malnutrition or obesity 2
- Diseases of skeletal muscle 2
- Paraplegia or quadriplegia 2
- Vegetarian diet 2
- Rapidly changing kidney function 2
- Calculating doses of potentially toxic drugs excreted by kidneys 2, 1
- Critical clinical decision-making requiring precise GFR 1
Special Populations
Pediatric Patients
Use pediatric-specific equations such as the Schwartz equation or Full Age Spectrum (FAS) equation—do not use adult equations. 1
Glomerular Disease
No eGFR equation has been specifically validated in glomerular disease patients. 1 Consider measured GFR for critical decisions, as hypoalbuminemia, glucocorticoids, and low muscle mass may affect estimates. 1
Elderly (>70 years)
The MDRD equation was not validated in persons older than 70 years. 2 The 2021 CKD-EPI equation has broader age validation but interpretation requires clinical context. 1
Clinical Interpretation
- eGFR <60 mL/min/1.73 m² for ≥3 months defines chronic kidney disease 3, 4
- eGFR <60 mL/min/1.73 m² represents loss of half or more of normal kidney function 3
- Consider drug dosage adjustments for renally excreted medications when eGFR <60 mL/min/1.73 m² 3
- Always interpret eGFR in clinical context with urinalysis and patient history 6