Calculate eGFR 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: The equation accounts for the relationship between cystatin C levels and glomerular filtration rate, with different coefficients for different demographic groups.

3. Importance of Cystatin C-Based eGFR

Details: Cystatin C-based eGFR is particularly useful in patients where creatinine-based estimates may be unreliable, such as those 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, age, sex, and race compared to creatinine, making it more reliable in certain patient populations.

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 testing preferred?
A: Cystatin C is particularly useful in elderly patients, those with malnutrition or obesity, 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, and inflammation, though it's generally more stable than creatinine.

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 estimates in many clinical situations.