What is the CKD (Chronic Kidney Disease)-EPI (Epidemiology Collaboration) creatinine equation for estimating glomerular filtration rate (GFR)?

The CKD-EPI Creatinine Equation for Estimating GFR

The 2009 CKD-EPI creatinine equation is the recommended equation for estimating glomerular filtration rate (GFR) in adults from serum creatinine levels measured by an assay calibrated to the isotope-dilution mass spectrometry reference method. 1

The CKD-EPI Creatinine Equation

The CKD-EPI creatinine equation is:

For females with serum creatinine (Scr) ≤ 0.7 mg/dL:

• eGFR = 144 × (Scr/0.7)^-0.329 × 0.993^Age

For females with Scr > 0.7 mg/dL:

• eGFR = 144 × (Scr/0.7)^-1.209 × 0.993^Age

For males with Scr ≤ 0.9 mg/dL:

• eGFR = 141 × (Scr/0.9)^-0.411 × 0.993^Age

For males with Scr > 0.9 mg/dL:

• eGFR = 141 × (Scr/0.9)^-1.209 × 0.993^Age

Where:

• eGFR is expressed in mL/min/1.73 m²
• Scr is serum creatinine in mg/dL
• Age is in years

Advantages of the CKD-EPI Equation

The CKD-EPI equation offers several advantages over previous equations:

• Improved accuracy: The CKD-EPI equation has less bias than the MDRD Study equation, especially at GFR ≥ 60 mL/min/1.73 m², with a small improvement in precision and greater overall accuracy 1
• Better performance at higher GFR levels: It enables reporting of numerical values across a wider range of GFRs 1
• Standardization: The equation requires serum creatinine measurements that are standardized to isotope-dilution mass spectrometry reference methodology 1

Clinical Implementation

When implementing the CKD-EPI equation in clinical practice:

• Clinical laboratories should report eGFR in addition to serum creatinine concentration and specify the equation used 1
• eGFR should be reported rounded to the nearest whole number and relative to a body surface area of 1.73 m² 1
• eGFR levels < 60 mL/min/1.73 m² should be reported as "decreased" 1

Limitations and Special Considerations

The CKD-EPI equation has some limitations:

• Accuracy in certain populations: The equation may be less accurate in certain clinical situations, including extremes of body size, severe malnutrition or obesity, skeletal muscle diseases, paraplegia, or vegetarian diet 1
• Confirmatory testing: For adults with eGFRcreat 45-59 mL/min/1.73 m² who do not have other markers of kidney damage, measuring cystatin C for confirmation of CKD may be useful 1
• Combined biomarkers: For increased accuracy, especially in cases where creatinine-based estimates may be unreliable, the combined creatinine-cystatin C equation (eGFRcr-cys) provides better performance 1, 2

Current Recommendations

The 2012 KDIGO guidelines and subsequent updates recommend:

1. Using the CKD-EPI creatinine equation for initial assessment of GFR 1
2. Understanding clinical settings in which eGFRcreat may be less accurate 1
3. Using additional tests (such as cystatin C or clearance measurement) for confirmatory testing when eGFR based on serum creatinine is less accurate 1
4. Avoiding the use of race in the computation of eGFR 1

Importance for Patient Outcomes

Accurate GFR estimation is critical for:

• Proper CKD staging and risk stratification
• Medication dosing adjustments
• Evaluation of kidney disease progression
• Clinical decision-making regarding interventions and referrals

The CKD-EPI equation helps identify patients at risk for adverse outcomes including mortality, cardiovascular events, and progression to kidney failure, allowing for earlier intervention and potentially improved outcomes 1.