Understanding the GFR Calculation and Glomerular Filtration Rate Formula
The Glomerular Filtration Rate (GFR) is widely considered the best overall index of kidney function in health and disease. Since measuring GFR directly is complex and requires specialized procedures, medical professionals typically use the Estimated GFR (eGFR). This calculation uses a mathematical formula derived from blood test results to estimate how well your kidneys are filtering waste.
This calculator utilizes the CKD-EPI 2021 Creatinine Equation, which is the currently recommended standard by the National Kidney Foundation and the American Society of Nephrology. It removes the race coefficient used in older formulas to ensure more equitable healthcare outcomes.
The GFR Formula Logic
The calculation depends primarily on three variables: Serum Creatinine levels, Age, and Biological Sex. The mathematical model changes based on whether the creatinine level is above or below a specific threshold (0.7 mg/dL for females, 0.9 mg/dL for males).
The variables:
Serum Creatinine (Scr): A waste product that comes from normal wear and tear on muscles of the body.
Age: GFR naturally declines with age.
Sex: Males and females have different muscle mass averages, affecting creatinine generation.
Interpreting Your GFR Results
The result is expressed in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73m²). This number generally helps determine the stage of Chronic Kidney Disease (CKD).
CKD Stage
eGFR Range
Description
Stage 1
90 or higher
Normal or high kidney function (if other signs of kidney damage are present).
Stage 2
60 – 89
Mildly decreased kidney function.
Stage 3a
45 – 59
Mild to moderately decreased kidney function.
Stage 3b
30 – 44
Moderate to severely decreased kidney function.
Stage 4
15 – 29
Severely decreased kidney function.
Stage 5
Below 15
Kidney failure (End Stage Renal Disease).
Why Monitoring GFR Matters
Early detection of a declining GFR allows for interventions that can slow or stop the progression of kidney disease. For example, if a 50-year-old male presents with a Serum Creatinine of 1.5 mg/dL, his eGFR would be approximately 56 mL/min/1.73m², placing him in Stage 3a. Identifying this allows doctors to adjust medications, manage blood pressure, and recommend dietary changes immediately.
Note: This calculator is for educational purposes only. Always consult a healthcare professional for diagnosis and treatment. Factors such as diet, hydration, and muscle mass can temporarily affect creatinine levels.
function calculateGFR() {
// Clear previous errors
document.getElementById('age-error').style.display = 'none';
document.getElementById('scr-error').style.display = 'none';
document.getElementById('gfr-result').style.display = 'none';
// Get Input Values
var age = parseFloat(document.getElementById('gfr-age').value);
var creatinine = parseFloat(document.getElementById('gfr-creatinine').value);
var gender = document.getElementById('gfr-gender').value;
// Validation
var hasError = false;
if (isNaN(age) || age <= 0) {
document.getElementById('age-error').style.display = 'block';
hasError = true;
}
if (isNaN(creatinine) || creatinine = 90) {
interpretation = "Stage 1: Normal or high kidney function.";
stageColor = "#28a745"; // Green
} else if (egfr >= 60) {
interpretation = "Stage 2: Mildly decreased kidney function.";
stageColor = "#8bc34a"; // Light Green
} else if (egfr >= 45) {
interpretation = "Stage 3a: Mild to moderately decreased kidney function.";
stageColor = "#ffc107"; // Yellow
} else if (egfr >= 30) {
interpretation = "Stage 3b: Moderate to severely decreased kidney function.";
stageColor = "#fd7e14"; // Orange
} else if (egfr >= 15) {
interpretation = "Stage 4: Severely decreased kidney function.";
stageColor = "#dc3545"; // Red
} else {
interpretation = "Stage 5: Kidney failure.";
stageColor = "#721c24"; // Dark Red
}
// Display Results
var resultBox = document.getElementById('gfr-result');
var valueDisplay = document.getElementById('gfr-value');
var interpDisplay = document.getElementById('gfr-interpretation');
valueDisplay.innerHTML = egfr + ' mL/min/1.73m²';
interpDisplay.innerHTML = interpretation;
valueDisplay.style.color = stageColor;
resultBox.style.display = 'block';
}