Blood urea nitrogen measured immediately before the start of the dialysis session
Blood urea nitrogen measured immediately after the end of the dialysis session
Enter your last session's URR to track improvement or decline over time
Enter BUN Values to Begin
Enter your pre-dialysis and post-dialysis BUN values to calculate URR and assess the adequacy of your hemodialysis session.
How to Use the URR Calculator
Enter Pre- and Post-Dialysis BUN Values
Select your BUN unit (mg/dL for US labs, mmol/L for European labs) then enter your pre-dialysis BUN from the blood draw taken before your session starts, and your post-dialysis BUN from the draw taken immediately when the session ends. Both values are required. Pre-BUN must be greater than post-BUN.
Review Your URR and Adequacy Status
The calculator instantly shows your URR percentage with a color-coded status: green (optimal, ≥70%), yellow (adequate, 65–70%), orange (borderline, 60–65%), or red (inadequate, <60%). The URR gauge shows your result against the KDOQI minimum and preferred target thresholds. A simplified Kt/V estimate is also displayed based on URR alone.
Unlock Full Kt/V with Advanced Mode
Click 'Show Advanced Kt/V Inputs' to enter your session duration (hours), total fluid removed (liters), and post-dialysis weight (kg). This enables the full Daugirdas second-generation spKt/V formula — the gold standard for dialysis adequacy used in KDOQI guidelines. Advanced mode also calculates equilibrated Kt/V, ultrafiltration rate safety assessment, and nPCR protein intake estimation.
Track Trends and Export Your Results
Enter your previous session's URR in the optional trend field to see whether your dialysis adequacy is improving or declining. Use the Export CSV button to save a complete session report for your medical records, or Print Report for a paper copy to bring to your nephrology appointment. Monthly tracking of URR and Kt/V helps your care team optimize your prescription.
Frequently Asked Questions
What is a good URR for hemodialysis?
According to KDOQI guidelines, a URR of 65% or higher meets the minimum standard for adequate hemodialysis. The preferred target is 70% or above, which provides a safety buffer to ensure the delivered dose consistently meets the minimum. A URR of 75% or higher is considered optimal by many nephrology centers. If your URR is between 60–65%, you are in the borderline zone and your nephrologist should review your prescription. URR below 60% indicates inadequate dialysis with significantly increased risk of morbidity and mortality, requiring prompt prescription adjustment — typically increasing session duration, blood flow rate, dialyzer surface area, or treatment frequency.
What is the difference between URR and Kt/V?
Both metrics measure dialysis adequacy using blood urea nitrogen as a marker, but they differ in complexity and accuracy. URR is the simpler measure — it is just the percentage reduction in BUN from pre to post dialysis. It requires only two lab values and is easy to calculate. Kt/V is more comprehensive: it accounts for urea generation during the session, convective clearance from fluid removal, and normalizes clearance to body water volume. The Daugirdas spKt/V formula corrects for these factors that URR ignores, making it more accurate. Most guidelines recommend reporting both values. When discrepancies exist between URR and Kt/V, the Kt/V result is generally more reliable because it accounts for more physiological variables.
How often should URR and Kt/V be measured?
CMS (Centers for Medicare and Medicaid Services) requires that dialysis adequacy — URR or Kt/V — be measured and documented for every dialysis patient at least once per month. This is a quality benchmark under the ESRD Quality Incentive Program. European guidelines from ERA-EDTA recommend measurement every 12 to 14 dialysis sessions, which corresponds roughly to monthly for a three-times-weekly patient. Measurement should also occur after any change in dialysis prescription, after a vascular access intervention or change, after any prolonged illness that may have altered body composition, or after a patient has been hospitalized. Monthly monitoring allows early detection of inadequate clearance before clinical symptoms worsen.
Why is my URR low even though my sessions seem long enough?
Several factors can reduce URR independently of session duration. Vascular access problems — including access recirculation, stenosis, or poor blood flow — are among the most common causes. If the actual blood flow through the dialyzer is lower than prescribed, urea clearance falls significantly. Other causes include dialyzer clotting or fibers, machine malfunction, blood pump speed errors, and early session termination due to symptoms. Patient-related factors include a higher-than-expected body water volume (V), which means the same amount of clearance produces a lower URR. High urea generation from high protein intake between sessions can also raise the pre-BUN, reducing the apparent URR. Your nephrologist and dialysis care team can review your treatment logs to identify the cause.
What is the ultrafiltration rate and why does it matter?
The ultrafiltration rate (UFR) is the speed at which fluid is removed from the body during dialysis, expressed in mL/kg/h. It is calculated by dividing total fluid removed (in mL) by the product of post-dialysis weight (kg) and session duration (hours). Research from the HEMO study and subsequent analyses has established that UFR above 13 mL/kg/h is associated with significantly increased cardiovascular events and mortality, independent of other factors. This finding has led to widespread clinical concern about overly rapid fluid removal. If your UFR is high, options include increasing session duration, reducing interdialytic weight gain through dietary sodium restriction and fluid intake control, or increasing treatment frequency. The calculator flags UFR above 13 mL/kg/h as a high-risk finding.
What is nPCR and what does it tell me?
Normalized Protein Catabolic Rate (nPCR) estimates dietary protein intake based on urea nitrogen appearance between dialysis sessions. It is calculated from the pre-dialysis BUN using the Daugirdas formula and reflects how much protein the patient is consuming relative to body weight. The target range is 1.0 to 1.4 g/kg/day for maintenance dialysis patients. Values below 1.0 g/kg/day may indicate protein malnutrition, poor dietary intake, or intercurrent illness — all associated with worse outcomes in dialysis patients. Values above 1.4 g/kg/day may indicate excessive protein or meat intake, which increases urea generation and can make it harder to achieve adequate URR with standard session parameters. nPCR is used in conjunction with serum albumin and clinical nutritional assessment to guide dietary counseling.