How is GFR Calculated

CKD-EPI Equation:

\[ eGFR = 142 \times \min(Scr/\kappa, 1)^\alpha \times \max(Scr/\kappa, 1)^{-1.200} \times 0.9938^{age} \times (1.012 \text{ if female}) \]

Estimated GFR (eGFR):

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1. What is GFR Calculation?

GFR (Glomerular Filtration Rate) calculation estimates kidney function by measuring how much blood passes through the glomeruli each minute. The CKD-EPI equation provides a standardized method for estimating GFR from serum creatinine levels and demographic factors.

2. How Does the Calculator Work?

The calculator uses the CKD-EPI equation:

\[ eGFR = 142 \times \min(Scr/\kappa, 1)^\alpha \times \max(Scr/\kappa, 1)^{-1.200} \times 0.9938^{age} \times (1.012 \text{ if female}) \]

Where:

\( Scr \) — Serum creatinine (mg/dL)
\( \kappa \) — 0.7 (female) or 0.9 (male)
\( \alpha \) — -0.241 (female) or -0.302 (male)
\( age \) — Age in years
1.012 — Female multiplier

Explanation: This equation accounts for the non-linear relationship between serum creatinine and glomerular filtration rate, with adjustments for age and gender differences in muscle mass and creatinine production.

3. Importance of GFR Calculation

Details: Accurate GFR estimation is essential for detecting kidney dysfunction, staging chronic kidney disease, monitoring disease progression, and adjusting medication dosages for drugs cleared by the kidneys.

4. Using the Calculator

Tips: Enter serum creatinine in mg/dL, age in years, and select gender. Ensure all values are valid with creatinine > 0 and age between 1-120 years for accurate results.

5. Frequently Asked Questions (FAQ)

Q1: What is the difference between measured and estimated GFR?
A: Measured GFR uses exogenous markers like inulin or iohexol, while eGFR estimates GFR using mathematical equations based on serum creatinine and demographic factors.

Q2: Why does age affect GFR calculation?
A: Kidney function naturally declines with age due to reduced nephron mass and changes in renal blood flow, which is accounted for in the equation.

Q3: How often should GFR be calculated?
A: Frequency depends on clinical context - annually for screening in at-risk populations, more frequently for monitoring known kidney disease or medication adjustments.

Q4: What factors can affect GFR accuracy?
A: Extreme muscle mass, dietary protein intake, certain medications, and conditions affecting creatinine metabolism can impact eGFR accuracy.

Q5: When is direct GFR measurement recommended?
A: Direct measurement is preferred when precise GFR is needed for drug dosing, in research settings, or when eGFR may be unreliable due to unusual body composition.