function calculateGFR() {
var scr = parseFloat(document.getElementById('scrInput').value);
var age = parseFloat(document.getElementById('ageInput').value);
var gender = document.getElementById('genderInput').value;
var resultBox = document.getElementById('resultBox');
var gfrDisplay = document.getElementById('gfrValue');
var stageDisplay = document.getElementById('stageIndicator');
// Validation
if (isNaN(scr) || isNaN(age) || scr <= 0 || age = 90) {
stageText = "Stage 1: Normal or high kidney function";
stageClass = "stage-1";
} else if (eGFR >= 60) {
stageText = "Stage 2: Mildly decreased kidney function";
stageClass = "stage-2";
} else if (eGFR >= 45) {
stageText = "Stage 3a: Mildly to moderately decreased";
stageClass = "stage-3";
} else if (eGFR >= 30) {
stageText = "Stage 3b: Moderately to severely decreased";
stageClass = "stage-3";
} else if (eGFR >= 15) {
stageText = "Stage 4: Severely decreased kidney function";
stageClass = "stage-4";
} else {
stageText = "Stage 5: Kidney failure";
stageClass = "stage-5";
}
// Display Results
resultBox.style.display = "block";
gfrDisplay.innerHTML = eGFR + ' mL/min/1.73m²';
stageDisplay.className = "stage-indicator " + stageClass;
stageDisplay.innerText = stageText;
}
Understanding the Glomerular Filtration Rate (GFR) Equation
The Glomerular Filtration Rate (GFR) is the single best indicator of how well your kidneys are functioning. It measures the volume of fluid filtered from the kidney's glomerular capillaries into the Bowman's capsule per unit of time. Because measuring GFR directly requires complex and lengthy procedures (like inulin clearance), medical professionals use equations to estimate it (eGFR) based on accessible blood test results.
The CKD-EPI 2021 Equation
This calculator utilizes the CKD-EPI 2021 Creatinine Equation, which is currently recommended by the National Kidney Foundation. Unlike older formulas (like MDRD or the 2009 CKD-EPI), the 2021 version does not include a coefficient for race, providing a more equitable assessment of kidney function across different populations.
The variables required for this calculation are:
Serum Creatinine (mg/dL): A waste product generated by muscle metabolism. Healthy kidneys filter this out of the blood.
Age: Kidney function naturally declines slightly with age.
Gender: Biological sex affects muscle mass and creatinine generation rates.
How to Calculate GFR Manually
While the math is complex, the underlying formula for the 2021 CKD-EPI equation is as follows:
Your eGFR result places you into one of several stages of Chronic Kidney Disease (CKD). It is important to note that a single low result does not confirm CKD; results must persist for at least three months.
eGFR (mL/min/1.73m²)
Stage
Description
90 or higher
Stage 1
Normal or high kidney function (CKD is only present if other signs like protein in urine exist).
60 – 89
Stage 2
Mildly decreased kidney function.
45 – 59
Stage 3a
Mildly to moderately decreased kidney function.
30 – 44
Stage 3b
Moderately to severely decreased kidney function.
15 – 29
Stage 4
Severely decreased kidney function.
Below 15
Stage 5
Kidney failure (End Stage Renal Disease).
Example Calculation
Let's look at an example to understand how the calculator works:
Scenario: A 60-year-old male with a Serum Creatinine level of 1.2 mg/dL.
Inputs: Age = 60, Gender = Male, Scr = 1.2.
Constants: Since gender is Male, κ = 0.9 and α = -0.302.
Ratio: Scr/κ = 1.2 / 0.9 = 1.333.
Calculation: Since 1.333 is greater than 1, we use the "max" part of the equation heavily.
Result: The calculated eGFR would be approximately 68 mL/min/1.73m².
This result would classify the patient in Stage 2, indicating mildly decreased kidney function, which is often normal for a 60-year-old.