Cystatin C GFR Calculator

CKD-EPI Cystatin C Equation:

\[ eGFR = 133 \times \min(Scys/0.8, 1)^{-0.499} \times \max(Scys/0.8, 1)^{-1.328} \times 0.996^{Age} \times (0.932 \text{ if female}) \]

Estimated GFR (eGFR):

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1. What is the CKD-EPI Cystatin C Equation?

The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) Cystatin C equation estimates glomerular filtration rate (GFR) from serum cystatin C, age, and sex. Cystatin C is less influenced by muscle mass than creatinine, providing an alternative marker for kidney function assessment.

2. How Does the Calculator Work?

The calculator uses the CKD-EPI Cystatin C equation:

\[ eGFR = 133 \times \min(Scys/0.8, 1)^{-0.499} \times \max(Scys/0.8, 1)^{-1.328} \times 0.996^{Age} \times (0.932 \text{ if female}) \]

Where:

\( Scys \) — Serum cystatin C (mg/L)
\( Age \) — Age in years
0.932 — Female multiplier

Explanation: The equation accounts for the relationship between cystatin C levels and GFR, with adjustments for age and gender. Cystatin C is produced at a constant rate by all nucleated cells and is freely filtered by the glomerulus.

3. Importance of Cystatin C GFR Calculation

Details: Cystatin C-based GFR estimation is particularly useful in patients where creatinine-based estimates may be unreliable, such as those with extremes of muscle mass, malnutrition, or amputations. It provides complementary information to creatinine-based estimates.

4. Using the Calculator

Tips: Enter serum cystatin C in mg/L, age in years, and select gender. All values must be valid (cystatin C > 0, age between 1-120). Cystatin C levels are typically measured using standardized immunoassays.

5. Frequently Asked Questions (FAQ)

Q1: Why use cystatin C instead of creatinine?
A: Cystatin C is less affected by muscle mass, diet, and inflammation, making it more reliable in elderly patients, those with malnutrition, or individuals with unusual muscle mass.

Q2: What are normal cystatin C values?
A: Normal serum cystatin C levels are typically 0.5-1.0 mg/L in healthy adults, though reference ranges may vary by laboratory and assay method.

Q3: When is cystatin C particularly useful?
A: In elderly patients, those with cirrhosis, HIV, transplantation, or when creatinine-based estimates are questionable due to muscle mass variations.

Q4: Are there limitations to cystatin C measurement?
A: Cystatin C can be affected by thyroid dysfunction, corticosteroid use, and inflammation. Standardization between different assay methods is also important.

Q5: Should cystatin C replace creatinine for GFR estimation?
A: Both markers provide complementary information. Many guidelines recommend using both creatinine and cystatin C for the most accurate GFR estimation, particularly when confirmation is needed.