How Do You Calculate the Glomerular Filtration Rate?
The Glomerular Filtration Rate (GFR) is the single best index of kidney function. It measures how much blood your kidneys filter each minute. Because directly measuring GFR is complex and requires specific substances (like inulin), medical professionals use an estimated GFR (eGFR) based on blood tests.
The Calculation Formula (CKD-EPI 2021)
The modern standard for calculating eGFR is the CKD-EPI 2021 Creatinine Equation. This updated formula removes race as a variable, providing a more equitable assessment of kidney health. The calculation relies on serum creatinine levels, age, and sex.
The result of the calculation classifies kidney function into stages of Chronic Kidney Disease (CKD).
Stage
eGFR Range
Description
Stage 1
90+
Normal or high kidney function
Stage 2
60-89
Mildly decreased kidney function
Stage 3a
45-59
Mild to moderate decrease
Stage 3b
30-44
Moderate to severe decrease
Stage 4
15-29
Severely decreased kidney function
Stage 5
<15
Kidney failure (requires dialysis or transplant)
function calculateGFR() {
// 1. Get Inputs
var scr = parseFloat(document.getElementById('scr').value);
var age = parseFloat(document.getElementById('age').value);
var gender = document.getElementById('gender').value;
var resultContainer = document.getElementById('result-container');
var gfrDisplay = document.getElementById('gfrResult');
var stageDisplay = document.getElementById('stageResult');
var stageDesc = document.getElementById('stageDesc');
// 2. Validation
if (isNaN(scr) || isNaN(age)) {
alert("Please enter valid numbers for Age and Serum Creatinine.");
return;
}
if (scr <= 0 || age = 90) {
stage = "Stage 1 (Normal)";
desc = "Kidney function is normal. If you have other signs of kidney damage (like protein in urine), this is Stage 1 CKD.";
stageColor = "#28a745";
} else if (eGFR >= 60) {
stage = "Stage 2 (Mild)";
desc = "Mild loss of kidney function. Usually requires observation and management of risk factors.";
stageColor = "#8bc34a"; // light green
} else if (eGFR >= 45) {
stage = "Stage 3a (Mild to Moderate)";
desc = "Mild to moderate loss of kidney function.";
stageColor = "#ffc107"; // yellow/orange
} else if (eGFR >= 30) {
stage = "Stage 3b (Moderate to Severe)";
desc = "Moderate to severe loss of kidney function.";
stageColor = "#fd7e14"; // orange
} else if (eGFR >= 15) {
stage = "Stage 4 (Severe)";
desc = "Severe loss of kidney function. Preparation for kidney replacement therapy may be needed.";
stageColor = "#dc3545"; // red
} else {
stage = "Stage 5 (Kidney Failure)";
desc = "Kidney failure. Dialysis or transplant is typically necessary.";
stageColor = "#721c24"; // dark red
}
// 6. Display Results
resultContainer.style.display = "block";
gfrDisplay.innerText = eGFR.toFixed(1);
stageDisplay.innerText = stage;
stageDisplay.style.color = stageColor;
resultContainer.style.borderLeftColor = stageColor;
stageDesc.innerText = desc;
}