eGFR Calculator Cystatin C

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, age, and sex. Cystatin C is an alternative biomarker to creatinine that may provide more accurate GFR estimation in certain populations, as it is less influenced by muscle mass and diet.

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.

3. Importance of eGFR Calculation

Details: Accurate GFR estimation using cystatin C is particularly valuable in patients where creatinine-based estimates may be unreliable, such as those with extremes of muscle mass, malnutrition, or rapidly changing 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). 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 other non-renal factors, making it potentially more accurate in elderly, malnourished, or muscular individuals.

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 and assay method.

Q3: When is cystatin C particularly useful?
A: Useful in patients with extremes of muscle mass, amputees, spinal cord injury, malnutrition, obesity, 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, inflammation, and some malignancies. Standardization across different assays is also important.

Q5: Can cystatin C and creatinine be combined?
A: Yes, combined equations using both biomarkers (CKD-EPI creatinine-cystatin C equation) may provide the most accurate GFR estimation in many clinical situations.