Cystatin C Creatinine Calculator

Combined Cystatin-Creatinine 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 Combined Cystatin-Creatinine Equation?

The combined cystatin-creatinine 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, especially in populations where creatinine-based estimates may be unreliable.

2. How Does the Calculator Work?

The calculator uses the combined cystatin-creatinine 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 equation is particularly valuable when creatinine-based estimates may be affected by factors like muscle mass, diet, or certain medical conditions. Cystatin C is less influenced by these factors, making the combined approach more 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 cystatin-creatinine equation?
A: The combined equation provides superior accuracy and precision compared to equations using either biomarker alone, reducing misclassification of kidney disease.

Q2: When is cystatin C particularly useful?
A: Cystatin C is valuable in elderly patients, those with extremes of muscle mass, amputees, vegetarians, and patients with liver disease or obesity.

Q3: What are normal cystatin C values?
A: Normal serum cystatin C levels are typically 0.5-1.0 mg/L in healthy adults, though reference ranges may vary by laboratory.

Q4: Are there limitations to this equation?
A: The equation may be less accurate in pregnant women, patients with thyroid dysfunction, those on glucocorticoid therapy, and populations not represented in the development cohort.

Q5: How does cystatin C compare to creatinine?
A: Cystatin C is produced at a constant rate by all nucleated cells and is less affected by muscle mass, diet, and inflammation compared to creatinine.