eGFR Creatinine Cystatin Calculator

Creatinine-Cystatin Combined 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 Creatinine-Cystatin Combined Equation?

The Creatinine-Cystatin Combined 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 marker alone, especially in populations with altered muscle mass or other conditions affecting creatinine production.

2. How Does the Calculator Work?

The calculator uses the Creatinine-Cystatin Combined 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 markers, providing a more robust estimate of kidney function across diverse patient populations.

3. Importance of Combined eGFR Calculation

Details: The combined creatinine-cystatin C equation offers improved accuracy and precision in GFR estimation, particularly in elderly patients, those with extremes of muscle mass, and populations where creatinine-based equations may be less reliable.

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 combined creatinine-cystatin equation?
A: The combined equation provides more accurate GFR estimation than either marker alone, with better performance across diverse patient populations and reduced bias in special populations.

Q2: What are the advantages of cystatin C?
A: Cystatin C is less influenced by muscle mass, diet, and race compared to creatinine, making it particularly useful in elderly, malnourished, or obese patients.

Q3: When should combined equation be preferred?
A: Consider using the combined equation when creatinine-based estimates are questionable, in elderly patients, those with extremes of body composition, or when confirmation of CKD diagnosis is needed.

Q4: Are there limitations to this equation?
A: The equation may be less accurate in pregnant women, patients with thyroid dysfunction, those on corticosteroids, or with rapidly changing kidney function.

Q5: How does this compare to CKD-EPI creatinine equation?
A: The combined equation generally provides more accurate GFR estimation, particularly in the range of 30-75 mL/min/1.73m², and reduces misclassification of CKD.