Combined Cystatin C Creatinine GFR Calculator

CKD-EPI 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 CKD-EPI Combined Equation?

The CKD-EPI 2021 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 biomarker alone, especially in populations with altered muscle mass or other conditions affecting creatinine production.

2. How Does the Calculator Work?

The calculator uses the CKD-EPI 2021 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 biomarkers, 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 is recommended when confirmatory testing is needed or when creatinine-based estimates may be less reliable (e.g., in elderly, obese, or malnourished patients).

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 C equation?
A: The combined equation provides more accurate GFR estimation than either biomarker alone, with improved precision and reduced bias across diverse populations.

Q2: When is cystatin C measurement recommended?
A: Cystatin C is particularly useful when creatinine-based estimates may be unreliable, such as in elderly patients, those with extremes of muscle mass, or when confirmatory testing is needed.

Q3: What are the advantages of cystatin C?
A: Cystatin C is less influenced by muscle mass, diet, and race compared to creatinine, making it a more consistent biomarker across different patient populations.

Q4: Are there limitations to the combined equation?
A: The equation may be less accurate in specific populations like pregnant women, amputees, or those with rapidly changing kidney function. Cystatin C measurements can also be affected by thyroid dysfunction and corticosteroid use.

Q5: How does this compare to creatinine-only equations?
A: The combined equation generally provides more accurate GFR estimates and is less likely to misclassify CKD stages, particularly in borderline cases.