Estimating Kidney Function using eGFR

Understanding eGFR Calculations

• The American Journal of Kidney Diseases recommends reporting both eGFRcys and eGFRcreat-cys whenever cystatin C is measured, to ensure accurate kidney function assessment in patients with borderline kidney function 1
• An eGFR of 70 could represent either normal kidney function (if creatinine-based) or mild reduction (if cystatin C-based), fundamentally altering CKD staging, as noted by Kidney International 2

Immediate Action Steps for Accurate eGFR Assessment

• The KDIGO guidelines recommend measuring cystatin C in adults with creatinine-based eGFR 45-59 mL/min/1.73 m² to confirm CKD, and using the combined creatinine-cystatin C equation for accurate assessment 3 4 2
• Patients should request both creatinine-based and cystatin C-based eGFR values from the laboratory to determine if there is concordance or discordance between the two methods 1

Clinical Significance of eGFR Discordance

• The combined creatinine-cystatin C equation (eGFRcreat-cys) provides a more accurate estimate of kidney function, with 94.9% accuracy within 30% of measured GFR, and should be used in cases of discordance 1
• The American Journal of Kidney Diseases notes that when eGFR values differ by more than 20% between methods, the combined equation is substantially more accurate than either alone 5 1

Recommended Next Steps for Healthcare Providers

• The American Journal of Kidney Diseases recommends ordering both serum creatinine and cystatin C simultaneously, and calculating all three eGFR values using the 2012 CKD-EPI equations 1 2
• If discordance exists (>20% difference), the combined equation (eGFRcreat-cys) should be used for clinical decision-making, as it demonstrates 84% accuracy (P30) versus 50% for creatinine alone and 73% for cystatin C alone in discordant cases 5 1

Cystatin C in Clinical Practice

Introduction to Cystatin C

• Cystatin C is largely independent of muscle mass, making it superior for patients with variable muscle mass, malnutrition, or extremes of body composition, according to the KDIGO guidelines 6

Estimating GFR

• The 2012 CKD-EPI cystatin C equation uses 0.8 mg/L as the reference point in its formula for estimating GFR, as reported by the American Journal of Transplantation 7, 8

CKD Diagnosis

• KDIGO 2024 guidelines recommend cystatin C-based equations specifically for patients with stage 3a CKD (eGFR 45-59 mL/min/1.73m²) and variable muscle mass to confirm CKD diagnosis, as summarized by Praxis Medical Insights 6