eGFR Cystatin C and SCR Calculator

CKD-EPI 2021 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):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the CKD-EPI 2021 Combined Equation?

The CKD-EPI 2021 combined equation estimates glomerular filtration rate (GFR) from 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, accounting for their different physiological determinants to provide a more robust GFR estimation across diverse populations.

3. Importance of eGFR Calculation

Details: Accurate GFR estimation using combined biomarkers is particularly valuable in clinical scenarios where creatinine-based estimates may be unreliable, such as in patients with extremes of muscle mass, malnutrition, or neuromuscular diseases.

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 patient populations.

Q2: When is cystatin C particularly useful?
A: Cystatin C is less influenced by muscle mass, making it valuable in elderly patients, those with malnutrition, amputees, or individuals with neuromuscular disorders.

Q3: What are the advantages over creatinine-only equations?
A: Combined equations reduce misclassification of CKD, provide more accurate staging, and are less affected by non-GFR determinants of creatinine.

Q4: Are there limitations to this equation?
A: The equation may be less accurate in specific populations like pregnant women, those with rapidly changing kidney function, or individuals with thyroid disorders affecting cystatin C levels.

Q5: How should biomarkers be measured?
A: Both creatinine and cystatin C should be measured using standardized assays. Avoid testing after meat-heavy meals or vigorous exercise for creatinine.