Cystatin C Creatinine eGFR Calculator

Combined Cystatin C and Creatinine CKD-EPI 2021 Equation:

\[ eGFR = 135 \times \min(Scr/\kappa, 1)^{\alpha} \times \max(Scr/\kappa, 1)^{-0.544} \times \min(Scys/0.8, 1)^{-0.323} \times \max(Scys/0.8, 1)^{-0.778} \times 0.9961^{Age} \times (0.963 \text{ if female}) \]

Serum Creatinine (Scr):

mg/dL

Serum Cystatin C (Scys):

mg/L

Age:

years

Gender:

Estimated GFR (eGFR):

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1. What is the Combined Cystatin C and Creatinine CKD-EPI 2021 Equation?

The CKD-EPI 2021 combined equation estimates glomerular filtration rate (GFR) using both serum creatinine and cystatin C, along with age and sex. This combined approach provides superior accuracy compared to equations using either biomarker alone, especially in populations with non-traditional muscle mass or other factors affecting creatinine production.

2. How Does the Calculator Work?

The calculator uses the CKD-EPI 2021 combined equation:

Where:

\( Scr \) — Serum creatinine (mg/dL)
\( Scys \) — Serum cystatin C (mg/L)
\( \kappa \) — 0.7 (female) or 0.9 (male)
\( \alpha \) — -0.219 (female) or -0.144 (male)
\( Age \) — Age in years
0.963 — Female multiplier

Explanation: This equation combines the strengths of both creatinine and cystatin C biomarkers, providing more accurate GFR estimation across diverse patient populations.

3. Importance of Combined eGFR Calculation

Details: The combined creatinine-cystatin C equation is recommended when enhanced accuracy is needed, particularly in patients with extremes of muscle mass, obesity, malnutrition, or when creatinine-based estimates may be unreliable.

4. Using the Calculator

Tips: Enter serum creatinine in mg/dL, serum cystatin C in mg/L, age in years, and select gender. All values must be valid (creatinine > 0, cystatin C > 0, age between 1-120).

5. Frequently Asked Questions (FAQ)

Q1: Why use combined creatinine-cystatin C equation?
A: The combined equation provides superior accuracy and precision compared to either biomarker alone, with less bias across different patient subgroups.

Q2: When is cystatin C measurement recommended?
A: Cystatin C is particularly useful in patients with altered muscle mass, amputees, vegetarians, obesity, malnutrition, or when creatinine-based estimates are questionable.

Q3: What are the advantages of cystatin C?
A: Cystatin C is less influenced by muscle mass, diet, and race compared to creatinine, providing a more stable biomarker of kidney function.

Q4: Are there limitations to the combined equation?
A: The equation may be less accurate in extremes of age, pregnancy, certain ethnic groups not well-represented in development cohorts, and patients with specific medical conditions affecting cystatin C production.

Q5: How does this compare to the 2012 CKD-EPI combined equation?
A: The 2021 equation was developed with more diverse populations and provides improved accuracy, particularly at higher GFR levels and in non-European populations.