Cystatin C and 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 an alternative marker to creatinine that may provide more accurate GFR estimation in certain populations.

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 different coefficients for different demographic groups.

3. Importance of Cystatin C in GFR Estimation

Details: Cystatin C is less influenced by muscle mass, diet, and age compared to creatinine, making it particularly useful in elderly patients, malnourished individuals, and those with extremes of muscle mass.

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).

5. Frequently Asked Questions (FAQ)

Q1: Why use cystatin C instead of creatinine?
A: Cystatin C is less affected by muscle mass, diet, age, and sex, providing more accurate GFR estimation in specific populations like the elderly, malnourished, or those with muscle wasting.

Q2: What are the advantages of cystatin C?
A: Better correlation with measured GFR, less variability, and more accurate classification of CKD stages compared to creatinine-based equations in many populations.

Q3: When is cystatin C particularly useful?
A: In elderly patients, individuals with extremes of muscle mass, vegetarians, and when creatinine-based estimates are questionable.

Q4: Are there limitations to cystatin C measurement?
A: Cystatin C levels can be affected by thyroid dysfunction, corticosteroid use, inflammation, and some malignancies.

Q5: Should cystatin C replace creatinine in GFR estimation?
A: Many guidelines recommend using both markers combined for the most accurate GFR estimation, especially when precise assessment is critical.