eGFR Creatinine Cystatin C Calculation Lab Test

CKD-EPI Creatinine-Cystatin C 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 Creatinine-Cystatin C Equation?

The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) 2021 creatinine-cystatin C equation estimates glomerular filtration rate (GFR) using both serum creatinine and serum cystatin C, along with age and sex. This combined equation provides superior accuracy compared to equations using either biomarker alone.

2. How Does the Calculator Work?

The calculator uses the CKD-EPI 2021 creatinine-cystatin C 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 advantages of both creatinine and cystatin C biomarkers, providing more accurate GFR estimation across diverse populations.

3. Importance of Combined Creatinine-Cystatin C eGFR

Details: The combined equation reduces bias and improves precision compared to single-biomarker equations. It's particularly valuable in populations where creatinine alone may be misleading, such as in individuals with unusual muscle mass, obesity, or chronic illness.

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 instead of creatinine alone?
A: The combined equation provides superior accuracy and reduces the limitations associated with either biomarker used alone, such as muscle mass effects on creatinine or inflammation effects on cystatin C.

Q2: What are the advantages of cystatin C over creatinine?
A: Cystatin C is less affected by muscle mass, diet, and race, making it more reliable in elderly, malnourished, or amputee patients.

Q3: When should combined creatinine-cystatin C testing be used?
A: Recommended when precise GFR estimation is needed for clinical decision-making, drug dosing, or when creatinine-based estimates are potentially inaccurate.

Q4: Are there limitations to this equation?
A: Less accurate in extremes of age, pregnant women, and those with rapidly changing kidney function or certain medical conditions affecting cystatin C production.

Q5: How does this compare to the 2012 CKD-EPI combined equation?
A: The 2021 equation was developed with more diverse populations and provides improved accuracy, especially at higher GFR levels.