Cystatin C and Creatinine GFR Calculator

Cystatin C and Creatinine GFR Equation:

\[ eGFR = 135 \times \min(Scr/\kappa, 1)^\alpha \times \max(Scr/\kappa, 1)^{-0.544} \times \min(Scys/0.8, 1)^{-0.323} \times \max(Scys/0.8, 1)^{-0.778} \times 0.9961^{Age} \times (0.963 \text{ if female}) \]

Serum Creatinine (Scr):

mg/dL

Serum Cystatin C (Scys):

mg/L

Age:

years

Gender:

Estimated GFR (eGFR):

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1. What is the Cystatin C and Creatinine GFR Equation?

The Cystatin C and Creatinine GFR equation estimates glomerular filtration rate (GFR) using both serum creatinine and serum cystatin C, along with age and sex. This combined approach provides superior accuracy compared to equations using either biomarker alone, especially in populations where creatinine-based estimates may be unreliable.

2. How Does the Calculator Work?

The calculator uses the Cystatin C and Creatinine GFR equation:

Where:

\( Scr \) — Serum creatinine (mg/dL)
\( Scys \) — Serum cystatin C (mg/L)
\( \kappa \) — 0.7 (female) or 0.9 (male)
\( \alpha \) — -0.219 (female) or -0.144 (male)
\( Age \) — Age in years
0.963 — Female multiplier

Explanation: This equation combines the strengths of both creatinine and cystatin C biomarkers, providing more accurate GFR estimation across diverse patient populations.

3. Importance of Combined Cystatin C and Creatinine GFR Calculation

Details: The combined equation is particularly valuable for patients with conditions that affect muscle mass (where creatinine may be misleading), obesity, or other factors that limit the accuracy of single-biomarker equations. It provides more reliable kidney function assessment for medication dosing and CKD staging.

4. Using the Calculator

Tips: Enter serum creatinine in mg/dL, serum cystatin C in mg/L, age in years, and select gender. All values must be valid (creatinine > 0, cystatin C > 0, age between 1-120).

5. Frequently Asked Questions (FAQ)

Q1: Why use both cystatin C and creatinine?
A: Combining both biomarkers reduces the limitations of each individual marker and provides more accurate GFR estimation across different patient populations.

Q2: When is this combined equation particularly useful?
A: In elderly patients, those with extremes of muscle mass, amputees, patients with neuromuscular diseases, and when precise GFR estimation is critical for drug dosing.

Q3: What are the advantages of cystatin C over creatinine?
A: Cystatin C is less influenced by muscle mass, diet, and race, and may detect early kidney dysfunction more sensitively.

Q4: Are there limitations to this equation?
A: The equation may be less accurate in pregnant women, patients with thyroid dysfunction, or those taking corticosteroids that affect cystatin C levels.

Q5: How does this compare to CKD-EPI creatinine equation?
A: The combined equation generally provides more accurate GFR estimation, especially in populations where creatinine-based equations are known to have limitations.