Glomerular Filtration Rate (GFR) is a crucial test used to assess how well your kidneys are functioning. It measures the amount of blood that your kidneys filter per minute. A normal GFR is generally considered to be 90 or higher, but kidney function can vary with age.
How is GFR Calculated?
The most common method for estimating GFR is by using formulas that incorporate serum creatinine levels along with other factors like age, gender, and race. These formulas provide an estimation because directly measuring GFR is complex and typically requires specialized tests.
The CKD-EPI Equation (2009)
This calculator uses the widely adopted Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, which is considered more accurate than older formulas like the Cockcroft-Gault equation, especially in individuals with normal or mildly impaired kidney function. The CKD-EPI equation is a piecewise linear regression model based on the following parameters:
Serum Creatinine (SCr): A waste product produced by muscles, filtered by the kidneys. Higher levels can indicate poorer kidney function.
Age: Kidney function naturally declines with age.
Gender: Differences in body composition influence creatinine levels.
Race: Historically, race has been a factor in the equation due to observed differences in muscle mass and creatinine production, though its inclusion is a subject of ongoing discussion and revision in clinical practice.
The Formula (Simplified Explanation of CKD-EPI 2009):
The CKD-EPI equation has different coefficients and constants depending on whether the serum creatinine is below or above a certain threshold, and whether the individual is male or female and identified as African American or not. The general structure involves:
Calculating a baseline GFR based on age, gender, and race.
Adjusting this baseline based on the measured serum creatinine level.
Using specific constants and factors (often denoted as 'k' and 'α') for different demographic groups.
For example, a simplified representation might involve terms like:
eGFR = 141 × min(SCr/κ, 1)^α × max(SCr/κ, 1)^-1.209 × 0.993^Age × (1.018 if female) × (1.159 if Black)
Where:
SCr is serum creatinine.
κ is 0.7 for females and 0.9 for males.
α is -0.329 for females and -0.411 for males.
min(SCr/κ, 1) means if SCr/κ is less than 1, use SCr/κ; otherwise, use 1.
max(SCr/κ, 1) means if SCr/κ is greater than 1, use SCr/κ; otherwise, use 1.
The exact values for κ and α, and the specific race adjustment, differ slightly within the CKD-EPI 2009 framework for different creatinine ranges and demographic groups. This calculator implements the standard 2009 CKD-EPI equation for accuracy.
Why is GFR Important?
Regular monitoring of GFR is essential for:
Detecting kidney disease early, often before symptoms appear.
Monitoring the progression of kidney disease.
Adjusting medication dosages, as many drugs are cleared by the kidneys.
Making decisions about treatment options, such as dialysis or transplantation.
Interpreting Your Estimated GFR (eGFR)
90 mL/min/1.73m² or higher: Generally considered normal kidney function.
60-89 mL/min/1.73m²: May indicate mild kidney damage or normal function that is declining with age.
Below 60 mL/min/1.73m²: Indicates kidney disease, and the lower the number, the more severe the disease.
Disclaimer: This calculator provides an estimation of GFR using the CKD-EPI equation. It is intended for informational purposes only and does not substitute for professional medical advice. Always consult with a healthcare provider for diagnosis, treatment, and interpretation of your kidney function results.
function calculateGFR() {
var creatinine = parseFloat(document.getElementById("creatinine").value);
var age = parseInt(document.getElementById("age").value);
var gender = document.getElementById("gender").value;
var race = document.getElementById("race").value;
var resultDiv = document.getElementById("result");
if (isNaN(creatinine) || isNaN(age) || creatinine <= 0 || age < 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for creatinine and age.";
return;
}
var raceFactor = 1;
if (race === "african american") {
raceFactor = 1.159;
}
var genderFactor = 1;
if (gender === "female") {
genderFactor = 1.018;
}
var a = -0.203; // Constant for all groups in CKD-EPI 2009
var b = -1.207; // Constant for all groups in CKD-EPI 2009
var c = 0; // Placeholder for adjustment based on creatinine ratio
var k = 0; // Placeholder for gender-specific constant
if (gender === "male") {
k = 0.9;
if (race === "african american") {
a = -0.203; // same as default for male
b = -1.207; // same as default for male
} else {
a = -0.203; // same as default for male
b = -1.207; // same as default for male
}
} else { // female
k = 0.7;
if (race === "african american") {
a = -0.279; // specific for female african american
b = -1.207; // same as default for female
} else {
a = -0.203; // specific for female non-african american
b = -1.207; // same as default for female
}
}
var scrRatio = creatinine / k;
if (scrRatio < 1) {
c = Math.pow(scrRatio, a);
} else {
c = Math.pow(scrRatio, b);
}
var gfr = 141 * c * Math.pow(0.993, age) * genderFactor * raceFactor;
// Ensure GFR is not negative, though unlikely with this formula
if (gfr < 0) gfr = 0;
resultDiv.innerHTML = "Estimated GFR: " + gfr.toFixed(2) + " mL/min/1.73m²";
}