Cystatin C Calculation

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 Cystatin C Equation?

The Cystatin C equation estimates glomerular filtration rate (GFR) from serum cystatin C levels, age, and sex. Cystatin C is an alternative biomarker to creatinine that may provide more accurate GFR estimation in certain populations, particularly those with reduced muscle mass or altered creatinine metabolism.

2. How Does the Calculator Work?

The calculator uses the 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 adjustments for age and gender. Cystatin C is less influenced by muscle mass than creatinine.

3. Importance of Cystatin C eGFR Calculation

Details: Cystatin C-based GFR estimation is particularly valuable in patients where creatinine-based equations may be inaccurate, such as elderly individuals, those with malnutrition, amputees, or people with liver disease.

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 other non-renal factors, making it more reliable in certain patient populations.

Q2: When is Cystatin C particularly useful?
A: In elderly patients, those with extremes of muscle mass, malnutrition, liver cirrhosis, or when creatinine-based estimates are questionable.

Q3: Are there limitations to Cystatin C measurement?
A: Cystatin C can be affected by thyroid dysfunction, glucocorticoid therapy, and inflammation. It's also more expensive than creatinine testing.

Q4: Can Cystatin C and creatinine be used together?
A: Yes, combined equations using both biomarkers may provide the most accurate GFR estimation in many clinical situations.

Q5: 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.