Glomerular Filtration Rate (GFR) Calculator
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How Do You Calculate Glomerular Filtration Rate
This is a complex medical calculation that typically requires specific laboratory test results and patient demographics. **The most common formula used is the CKD-EPI creatinine equation, which is generally preferred over the MDRD study equation for estimating GFR.**
To accurately calculate your Glomerular Filtration Rate (GFR), you will need the following information, typically provided by your doctor from a blood test:
* **Serum Creatinine:** This is a waste product produced by muscles that is filtered out by the kidneys. It's measured in mg/dL.
* **Age:** Your age in years.
* **Sex:** Biological sex (Male or Female).
* **Race:** While historically included, race is being phased out of GFR calculations due to concerns about racial bias and scientific validity. However, some older equations or specific clinical contexts might still use it. For the purpose of this calculator, we will use a common version of the CKD-EPI equation that does not rely on race.
**Understanding Glomerular Filtration Rate (GFR)**
The Glomerular Filtration Rate (GFR) is a crucial indicator of kidney function. It measures how well your kidneys are filtering waste products and excess fluid from your blood. The kidneys contain millions of tiny filtering units called glomeruli. GFR estimates the volume of fluid that is filtered by these glomeruli each minute.
A normal GFR generally indicates healthy kidneys. As kidney function declines, the GFR decreases. Doctors use GFR measurements to:
* **Diagnose kidney disease:** A low GFR is a primary sign of chronic kidney disease (CKD).
* **Stage kidney disease:** CKD is staged from GFR levels, with Stage 1 being the mildest (normal or high GFR with kidney damage) and Stage 5 being kidney failure.
* **Monitor disease progression:** Tracking GFR over time helps assess how quickly kidney function is declining and whether treatments are effective.
* **Guide treatment decisions:** GFR levels influence medication dosages and decisions about dialysis or transplantation.
**The CKD-EPI Creatinine Equation (2009)**
The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation is widely used because it's considered more accurate than previous formulas, especially at higher GFR levels.
For **Males**:
GFR = 141 × min(Scr/0.9, 1)^-1.209 × 0.993^Age × 1.18 (if Black) + 1.18 (if Female)
*Note: The 'if Black' and 'if Female' modifiers are simplified/omitted for this calculator to avoid racial/gender bias, focusing on the core calculation.*
For **Females**:
GFR = 141 × min(Scr/0.7, 1)^-1.209 × 0.993^Age × 1.18 (if Black) + 1.18 (if Female)
*Note: The 'if Black' and 'if Female' modifiers are simplified/omitted for this calculator to avoid racial/gender bias, focusing on the core calculation.*
**Simplified CKD-EPI Equation (without race/gender modifiers for general use):**
The formula is complex and uses specific constants. For simplicity in this calculator, we'll use a version that accounts for the creatinine-to-eGFR relationship based on sex.
* **For Males:** If serum creatinine (Scr) ≤ 0.9, GFR = 133 × (Scr/0.9)^-0.297 × 0.993^Age
If serum creatinine (Scr) > 0.9, GFR = 133 × (Scr/0.9)^-1.209 × 0.993^Age
* **For Females:** If serum creatinine (Scr) ≤ 0.7, GFR = 166 × (Scr/0.7)^-0.297 × 0.993^Age
If serum creatinine (Scr) > 0.7, GFR = 166 × (Scr/0.7)^-1.209 × 0.993^Age
**Important Disclaimer:** This calculator is for educational purposes only. It is NOT a substitute for professional medical advice. Always consult with your doctor for diagnosis and treatment of kidney conditions.
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function calculateGFR() {
var serumCreatinine = parseFloat(document.getElementById("serumCreatinine").value);
var age = parseInt(document.getElementById("age").value);
var sex = document.getElementById("sex").value;
var gfr = 0;
if (isNaN(serumCreatinine) || isNaN(age) || serumCreatinine <= 0 || age <= 0) {
document.getElementById("result").innerHTML = "Error: Please enter valid positive numbers for Serum Creatinine and Age.";
return;
}
var scrFactor = 0;
var slope = 0;
var intercept = 0;
if (sex === "male") {
scrFactor = 0.9;
slope = -1.209;
intercept = 141; // CKD-EPI 2009 base constant for males
} else { // female
scrFactor = 0.7;
slope = -1.209;
intercept = 141; // CKD-EPI 2009 base constant for females (modified by gender factor later)
}
var scrValue = serumCreatinine / scrFactor;
var ageFactor = Math.pow(0.993, age);
var k = (sex === "male") ? 1.41 : 1.018; // CKD-EPI 2009 gender constant
var adjustment = (sex === "male") ? 1.018 : 1; // CKD-EPI 2009 adjustment constant. Replaced gender factor application.
// Using simplified CKD-EPI 2009 equation structure
var gfrNumerator = intercept * ageFactor;
var gfrDenominator = Math.pow(scrValue, slope);
gfr = gfrNumerator / gfrDenominator;
// Applying gender adjustment and potential race factor (simplified)
if (sex === "female") {
gfr = gfr * 0.742; // CKD-EPI 2009 gender factor for females
}
// The original 2009 CKD-EPI had a race factor, but current recommendations phase this out.
// For simplicity and modern practice, we are omitting a race factor here.
// If you were to include it (e.g., for older protocols):
// if (race === "black") {
// gfr = gfr * 1.212;
// }
// Ensure GFR is not negative (shouldn't happen with valid inputs but good practice)
gfr = Math.max(0, gfr);
var resultText = "Your estimated GFR is: " + gfr.toFixed(2) + " mL/min/1.73 m²";
document.getElementById("result").innerHTML = resultText;
}