Cystatin C eGFR 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 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 when muscle mass is abnormal.

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
\( -0.499 \) — Exponent for min part
\( -1.328 \) — Exponent for max part
\( age \) — Age in years
\( 0.996 \) — Age adjustment factor
\( 0.932 \) — Female multiplier

Explanation: The equation models 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 is particularly valuable in patients with abnormal muscle mass, malnutrition, obesity, or when creatinine-based estimates may be unreliable. It provides an independent assessment of 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 should be measured using standardized assays.

5. Frequently Asked Questions (FAQ)

Q1: Why use cystatin C instead of creatinine?
A: Cystatin C is less influenced by muscle mass, diet, and race compared to creatinine, making it more reliable in patients with altered muscle mass or when 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: In elderly patients, those with malnutrition, amputees, bodybuilders, patients with liver disease, or when creatinine-based eGFR is inconsistent with clinical assessment.

Q4: Are there limitations to cystatin C measurement?
A: Cystatin C can be affected by thyroid dysfunction, corticosteroid use, and inflammation. Standardization across different assay methods is also important.

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