Calculate GFR Using 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 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 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)
\( 0.8 \) — Reference value for cystatin C
\( -0.499 \) — Coefficient for the minimum part
\( -1.328 \) — Coefficient for the maximum part
\( age \) — Age in years
\( 0.996 \) — Age adjustment factor
\( 0.932 \) — Female multiplier

Explanation: This equation accounts for the relationship between cystatin C levels and GFR, with adjustments for age and gender differences in cystatin C metabolism.

3. Importance Of Cystatin C-Based GFR Calculation

Details: Cystatin C-based GFR estimation is particularly valuable in patients where creatinine-based equations may be less accurate, such as elderly individuals, those with malnutrition, amputees, or people with extreme muscle mass. It provides an independent assessment of kidney function that complements creatinine-based measurements.

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 levels are typically measured using standardized immunoassays.

5. Frequently Asked Questions (FAQ)

Q1: Why use cystatin C instead of creatinine?
A: Cystatin C is less influenced by muscle mass, diet, and other non-renal factors, making it more reliable in populations where creatinine may be misleading.

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: Cystatin C is especially valuable in elderly patients, those with cirrhosis, malnutrition, obesity, or when creatinine-based estimates are questionable.

Q4: Are there limitations to cystatin C measurement?
A: Cystatin C can be affected by thyroid dysfunction, glucocorticoid therapy, and inflammation. It may also be more expensive than creatinine testing.

Q5: Should cystatin C replace creatinine testing?
A: Cystatin C is often used as a complementary test rather than a replacement. Combined creatinine-cystatin C equations may provide the most accurate GFR estimation.