What is the estimated Glomerular Filtration Rate (eGFR) of a patient using the CKD-EPI equation?

Calculating eGFR Using the Most Recent 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, 2, 3

Primary Calculation Method: CKD-EPI 2021 (Race-Free)

The 2021 CKD-EPI equation represents the current gold standard, replacing older equations that included race as a variable. 1, 2 This equation was specifically developed to eliminate racial coefficients while maintaining accuracy across diverse populations. 1

Required Variables:

• Serum creatinine (must be IDMS-calibrated and traceable to international reference standards) 2, 3
• Age (in years) 2, 3
• Sex (male or female) 2, 3

Key Formula Details:

The equation uses: GFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^-1.200 × 0.9938^age × 1.012 [if female], where κ = 0.7 (females) or 0.9 (males), and α = -0.241 (females) or -0.302 (males). 1, 2

Why This Equation Supersedes Older Methods

Advantages Over MDRD:

• More accurate at GFR >60 mL/min/1.73 m² (MDRD is not validated above this threshold) 1, 2
• Less bias (median difference 2.5 vs. 5.5 mL/min/1.73 m²) 4
• Better precision (IQR 16.6 vs. 18.3 mL/min/1.73 m²) 4
• Greater accuracy (84.1% vs. 80.6% of estimates within 30% of measured GFR) 4

Advantages Over Cockcroft-Gault:

• Does not require body weight (eliminates errors from obesity or edema) 2, 5
• More accurate in CKD stages 3-5 (Cockcroft-Gault overestimates kidney function) 2, 5
• Standardized to body surface area (1.73 m²) 2

When to Add Cystatin C for Confirmatory Testing

Measure cystatin C and use the combined CKD-EPI creatinine-cystatin C equation (2021, race-free) when: 1, 2, 3

• eGFRcr is 45-59 mL/min/1.73 m² without other markers of kidney damage (to confirm or exclude CKD) 1, 3, 5
• Extremes of muscle mass (very high or very low) 2, 3
• Advanced cirrhosis 2, 3
• Cancer diagnosis 2, 3
• Dietary factors affecting creatinine (vegetarian diet, creatine supplements) 3

The combined creatinine-cystatin C equation provides the most accurate GFR estimates overall and reduces differential bias between populations. 1, 5

Critical Laboratory Requirements

Serum creatinine must be: 2, 3

• Measured using IDMS-calibrated assays 2, 3
• Traceable to international reference standards 1, 3
• Reported to two decimal places when <1 mg/dL 2, 3

Cystatin C (when used) must be: 1, 3

• Measured using assays traceable to international standard reference material 1
• Reported to the nearest hundredth (0.01 mg/L) 1

When to Measure GFR Directly

Consider measured GFR using exogenous markers (iohexol, radioisotopic clearance) when: 1, 2, 3

• Precise GFR measurement is critical for treatment decisions (e.g., chemotherapy dosing, living kidney donation) 1, 2
• Extremes of age or body size 2
• Calculating doses of potentially toxic drugs excreted by kidneys 2
• Acute kidney injury (all eGFR equations are invalid in non-steady-state) 1
• Preemptive renal transplant listing decisions 1

Special Population Considerations

Children:

Use pediatric-specific equations: 1, 2

• Schwartz equation and its modifications 1, 2
• Full Age Spectrum (FAS) equation 1, 2

Glomerular Disease:

No eGFR equation has been specifically validated for nephrotic syndrome or glomerular diseases. 1 Be aware that: 1

• Hypoalbuminemia may overestimate true GFR (increased tubular creatinine secretion) 1
• Glucocorticoids may increase serum cystatin C (underestimating eGFR) 1
• Low muscle mass overestimates eGFR with creatinine-based formulas 1

Common Pitfalls to Avoid

Do not use race coefficients - The 2021 equations eliminated race variables to reduce health disparities and improve equity. 1, 5 Using older equations with race coefficients (2009 CKD-EPI) can lead to underestimation of kidney disease in Black patients by approximately 10.8 mL/min/1.73 m². 1

Do not rely on serum creatinine alone - Always use an estimating equation; raw creatinine values do not account for age, sex, or muscle mass. 2, 3

Do not use eGFR in acute kidney injury - All equations require steady-state creatinine levels. 1

Do not use MDRD for eGFR >60 mL/min/1.73 m² - It systematically underestimates at higher GFR values. 1, 4

Practical Implementation

Request that your laboratory automatically reports eGFR using the 2021 CKD-EPI equation (race-free) whenever serum creatinine is ordered. 2, 3 The equation should be applied consistently within your geographical region for standardization. 3