GFR Calculator Using Cystatin C

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 and diet compared to 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: This equation provides a more accurate estimation of GFR using cystatin C as a filtration marker, particularly useful when creatinine-based estimates may be unreliable.

3. Importance of Cystatin C-Based eGFR

Details: Cystatin C-based GFR estimation is valuable in populations where creatinine may be misleading, such as elderly patients, those with reduced muscle mass, amputees, or individuals with unusual 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, age, sex, and diet, making it more reliable in certain populations where creatinine may give misleading results.

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.

Q3: When is cystatin C particularly useful?
A: Useful in elderly patients, those with malnutrition, liver disease, amputees, bodybuilders, and when creatinine-based estimates are questionable.

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

Q5: Can both creatinine and cystatin C be used together?
A: Yes, combined creatinine-cystatin C equations exist and may provide the most accurate GFR estimation in many clinical situations.