Creatinine Cystatin C GFR Calculator

Creatinine Cystatin C 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 Creatinine Cystatin C GFR Equation?

The Creatinine Cystatin C GFR equation estimates glomerular filtration rate (GFR) using both serum creatinine and serum cystatin C, along with age and sex. This combined approach provides a more accurate assessment of kidney function than equations using either biomarker alone, as it accounts for limitations of individual markers.

2. How Does the Calculator Work?

The calculator uses the Creatinine Cystatin C 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 a more robust estimate of kidney function that is less affected by factors like muscle mass or inflammation.

3. Importance of Combined Creatinine and Cystatin C GFR Calculation

Details: The combined creatinine-cystatin C equation provides superior accuracy compared to equations using either biomarker alone. It is particularly valuable in populations where creatinine-based estimates may be unreliable, such as in elderly individuals, those with extremes of muscle mass, or patients with chronic conditions affecting creatinine production.

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). Use standardized laboratory measurements for both biomarkers.

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 populations where creatinine production is affected by factors like muscle mass, diet, or chronic illness.

Q2: What are the advantages of cystatin C over creatinine?
A: Cystatin C is less influenced by muscle mass, diet, and sex differences, making it particularly useful in elderly patients, those with malnutrition, or individuals with amputations.

Q3: When should combined creatinine-cystatin C testing be considered?
A: When creatinine-based estimates are potentially unreliable, for confirmation of CKD diagnosis, for accurate drug dosing decisions, or when precise GFR estimation is clinically critical.

Q4: Are there limitations to this equation?
A: The equation may be less accurate in extremes of age, pregnant women, those with rapidly changing kidney function, or patients with conditions affecting cystatin C metabolism (e.g., thyroid disorders, steroid use).

Q5: How does this compare to measured GFR?
A: While more accurate than creatinine-only equations, the combined equation still represents an estimate. For precise clinical decisions, measured GFR (e.g., via iohexol clearance) remains the gold standard.