Cystatin C GFR Calculation

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: This equation uses cystatin C as the filtration marker, which is less influenced by muscle mass and diet compared to creatinine.

3. Importance of Cystatin C GFR Calculation

Details: Cystatin C-based GFR estimation is particularly useful when creatinine-based estimates may be unreliable, such as in patients with extremes of muscle mass, amputations, or specific dietary patterns.

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, and race, providing more accurate GFR estimation in certain clinical scenarios.

Q2: What are the advantages of cystatin C?
A: Better correlation with measured GFR, less variability, and improved accuracy in elderly patients and those with reduced muscle mass.

Q3: When should cystatin C be measured?
A: When creatinine-based estimates are questionable, in patients with extremes of muscle mass, or when more precise GFR estimation is needed.

Q4: Are there limitations to cystatin C measurement?
A: Cystatin C levels can be affected by thyroid dysfunction, corticosteroids, and inflammation. It's also more expensive than creatinine.

Q5: Can both creatinine and cystatin C be used together?
A: Yes, combined equations using both markers provide the most accurate GFR estimation and are recommended when both values are available.