Cystatin C eGFR Calc

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 eGFR Equation?

The Cystatin C equation estimates glomerular filtration rate (GFR) from serum cystatin C, 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 production.

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 kidney function, with adjustments for age and gender differences in cystatin C production.

3. Importance of Cystatin C eGFR Calculation

Details: Cystatin C-based eGFR provides an alternative assessment of kidney function that is less influenced by muscle mass, diet, and other non-renal factors that affect creatinine levels. It is particularly useful in elderly patients, patients with malnutrition, amputees, and those 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). Cystatin C measurement should be performed using standardized assays.

5. Frequently Asked Questions (FAQ)

Q1: Why use Cystatin C instead of creatinine?
A: Cystatin C is less affected by muscle mass, diet, and age compared to creatinine, making it more reliable in populations where these factors are variable.

Q2: What are the advantages of Cystatin C?
A: Better correlation with measured GFR, less inter-individual variation, and less dependence on demographic factors compared to creatinine-based equations.

Q3: When is Cystatin C particularly useful?
A: In elderly patients, patients with extremes of muscle mass (obesity, malnutrition), liver cirrhosis, 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. Standardization between different assays is also important.

Q5: Can Cystatin C and creatinine be used together?
A: Yes, combined equations using both biomarkers (CKD-EPI creatinine-cystatin C equation) often provide the most accurate GFR estimation.