GFR Calculated From 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 an alternative filtration marker that is less influenced by muscle mass than creatinine, providing complementary information about kidney function.

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 models the non-linear relationship between cystatin C and GFR, with different coefficients for different demographic groups.

3. Importance of Cystatin C-Based GFR Calculation

Details: Cystatin C-based GFR estimation is particularly valuable in patients where creatinine-based estimates may be unreliable, such as those with extremes of muscle mass, malnutrition, or amputations. It provides an independent assessment of kidney function.

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 influenced by muscle mass, diet, and ethnicity, making it more reliable in specific populations like the elderly, malnourished, or those with muscle wasting.

Q2: When is cystatin C particularly useful?
A: In patients with extremes of muscle mass, amputees, spinal cord injury, malnutrition, obesity, and when creatinine-based estimates are questionable.

Q3: What are the limitations of cystatin C?
A: Cystatin C can be affected by thyroid dysfunction, corticosteroid use, inflammation, and some malignancies. It's also more expensive than creatinine testing.

Q4: Can cystatin C and creatinine be used together?
A: Yes, combining both markers (CKD-EPI creatinine-cystatin C equation) often provides the most accurate GFR estimation.

Q5: What is the normal range for cystatin C?
A: Typically 0.5-1.0 mg/L in healthy adults, though reference ranges may vary by laboratory and population.