The Ultrafiltration (UF) Rate is a critical parameter in hemodialysis prescriptions. It determines the speed at which fluid is removed from the patient's blood during a dialysis session. Calculating the correct UF rate is essential for balancing effective fluid removal with patient safety and cardiovascular stability.
What is Ultrafiltration?
In the context of renal replacement therapy, ultrafiltration is the movement of fluid across a semipermeable membrane as a result of a pressure gradient (transmembrane pressure). While diffusion removes toxins like urea and creatinine, ultrafiltration is primarily responsible for removing excess water that has accumulated in the body between treatments.
How to Calculate UF Rate
The calculation involves determining the total volume of fluid that needs to be removed and dividing it by the duration of the treatment. The basic formula is:
Pre-Weight: The patient's weight immediately before starting the session.
Target Dry Weight: The prescribed weight the patient should achieve by the end of the session (estimated weight without excess fluid).
Additional Fluids: Any volume added during treatment (e.g., priming saline, rinseback, oral intake, or IV antibiotics) must also be removed.
Why Normalize by Weight (mL/kg/hr)?
While the raw UF rate (mL/hr) is useful for setting the machine, clinical studies suggest that the weight-adjusted UF rate is a better predictor of patient outcomes. This is calculated by dividing the hourly rate by the patient's dry weight.
High UF rates (generally defined as > 10-13 mL/kg/hr) are associated with:
Intradialytic hypotension (low blood pressure during treatment).
Nephrology guidelines generally recommend keeping the UF rate as low as reasonably possible. If the calculated rate exceeds safety thresholds (e.g., 13 mL/kg/hr), clinicians may consider:
Extending the treatment time.
Increasing the frequency of sessions.
Re-evaluating the target dry weight.
Counseling the patient on restricting interdialytic fluid intake.
function calculateUFRate() {
// 1. Get DOM elements
var preWeightInput = document.getElementById("ufPreWeight");
var targetWeightInput = document.getElementById("ufTargetWeight");
var extraFluidInput = document.getElementById("ufExtraFluid");
var timeInput = document.getElementById("ufTime");
var resultDiv = document.getElementById("ufResults");
var resTotal = document.getElementById("resTotalVolume");
var resHourly = document.getElementById("resHourlyRate");
var resNormalized = document.getElementById("resNormalizedRate");
var safetyWarning = document.getElementById("ufSafetyWarning");
// 2. Parse values
var preWeight = parseFloat(preWeightInput.value);
var targetWeight = parseFloat(targetWeightInput.value);
var extraFluid = parseFloat(extraFluidInput.value);
var time = parseFloat(timeInput.value);
// 3. Validation
if (isNaN(preWeight) || isNaN(targetWeight) || isNaN(time)) {
alert("Please enter valid numbers for Weight and Time.");
return;
}
if (time <= 0) {
alert("Treatment duration must be greater than 0.");
return;
}
// Handle empty extra fluid as 0
if (isNaN(extraFluid)) {
extraFluid = 0;
}
// 4. Logic Calculation
// Weight difference in kg (1 kg water = 1000 mL)
var weightDiffKg = preWeight – targetWeight;
// Convert weight diff to mL
var weightDiffmL = weightDiffKg * 1000;
// Total Volume to remove (Weight gain + Extra fluid inputs)
var totalVolumeML = weightDiffmL + extraFluid;
// Ensure we aren't removing negative fluid (unless intended, but usually implies error in UF context)
if (totalVolumeML 13 mL/kg/hr is a common threshold for high mortality risk
// > 10 mL/kg/hr is a warning zone
if (normalizedRate > 13) {
safetyWarning.innerHTML = "⚠️ High Risk: Rate exceeds 13 mL/kg/hr. Consider extending time.";
safetyWarning.style.color = "#e74c3c"; // Red
} else if (normalizedRate > 10) {
safetyWarning.innerHTML = "⚠️ Caution: Rate exceeds 10 mL/kg/hr.";
safetyWarning.style.color = "#f39c12"; // Orange
} else {
safetyWarning.innerHTML = "✅ Rate is within standard safety limits (<10 mL/kg/hr).";
safetyWarning.style.color = "#27ae60"; // Green
}
}