Cystatin C Creatinine GFR Calculator

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 CKD-EPI 2021 Equation?

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

2. How Does the Calculator Work?

The calculator uses the CKD-EPI 2021 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: The equation combines the advantages of both creatinine and cystatin C biomarkers, providing a more robust estimate of kidney function across diverse patient populations.

3. Importance of Combined Creatinine and Cystatin C GFR Calculation

Details: Using both creatinine and cystatin C improves accuracy in patients with extremes of muscle mass, obesity, malnutrition, or other conditions where creatinine alone may be misleading. This combined approach is recommended in clinical guidelines for more precise GFR estimation.

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 instead of creatinine alone?
A: The combined equation reduces bias and improves accuracy, especially in elderly patients, those with extremes of body composition, and populations where creatinine production may be altered.

Q2: What are the advantages of cystatin C over creatinine?
A: Cystatin C is less influenced by muscle mass, diet, and race compared to creatinine, making it a more reliable biomarker in many clinical scenarios.

Q3: When should combined creatinine-cystatin C testing be considered?
A: When confirmatory testing is needed, in patients with conditions affecting muscle mass, for medication dosing decisions, and when precise GFR estimation is critical.

Q4: Are there limitations to this equation?
A: The equation may be less accurate in pregnant women, patients with certain inflammatory conditions affecting cystatin C levels, and those with rapidly changing kidney function.

Q5: How does this compare to the 2009 CKD-EPI creatinine equation?
A: The 2021 combined equation generally provides more accurate GFR estimates across the entire range of kidney function and reduces misclassification of CKD stages.