Creatinine Cystatin C Calculator

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 cystatin C biomarkers, along with age and sex. This combined approach provides superior accuracy compared to equations using either biomarker alone.

2. How Does the Calculator Work?

The calculator uses the CKD-EPI 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 strengths of both creatinine and cystatin C biomarkers, providing a more robust estimate of kidney function across diverse populations.

3. Importance of Combined eGFR Calculation

Details: Using both creatinine and cystatin C reduces the limitations associated with either biomarker alone, such as muscle mass effects on creatinine or inflammation effects on cystatin C. This combined approach is particularly valuable for patients with extremes of muscle mass, obesity, or chronic illnesses.

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 more accurate GFR estimation, especially in patients where creatinine or cystatin C alone may be misleading due to non-GFR determinants.

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

Q3: When should combined testing be considered?
A: When creatinine-based eGFR is potentially inaccurate (extremes of age/body composition), for confirmatory testing, or when precise GFR estimation is critical for clinical decisions.

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

Q5: How does this compare to measured GFR?
A: The combined creatinine-cystatin C equation correlates well with measured GFR and is often used as a reliable alternative when direct measurement is not feasible.