How to Calculate GFR with 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)
\( Age \) — Age in years
0.932 — Female multiplier

Explanation: This equation accounts for the inverse relationship between cystatin C levels and kidney function, with adjustments for age and gender differences in cystatin C production.

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, paralysis, or amputations.

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 inflammatory conditions, making it more reliable in patients with altered muscle mass or nutritional status.

Q2: What are normal cystatin C levels?
A: Normal 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: In elderly patients, those with malnutrition, obesity, liver disease, or when creatinine-based estimates are inconsistent with clinical findings.

Q4: Are there limitations to cystatin C measurement?
A: Cystatin C can be affected by thyroid dysfunction, corticosteroid use, and some inflammatory conditions. It may also be more expensive than creatinine testing.

Q5: Can cystatin C and creatinine be combined?
A: Yes, combined equations using both biomarkers (creatinine-cystatin C equations) often provide the most accurate GFR estimates and are recommended in clinical guidelines.