Calculate urea reduction ratio (URR) and Kt/V (Daugirdas) for hemodialysis adequacy assessment. Includes eKt/V, UF rate monitoring, KDOQI reference bands, and common low-clearance review areas.
Dialysis adequacy — measured by the urea reduction ratio (URR) and Kt/V — is a core part of hemodialysis quality assessment. URR is a simple percentage reflecting how much blood urea nitrogen is removed during a session, while Kt/V (clearance × time / volume) provides a more physiologically rigorous measure that accounts for ultrafiltration and urea generation. KDOQI discusses a minimum spKt/V of 1.2 (target 1.4) and minimum URR of 65% (target 70%) for standard thrice-weekly hemodialysis.
This calculator uses the Daugirdas second-generation formula for single-pool Kt/V (spKt/V), which is the standard method used in clinical practice and quality reporting. It also provides the equilibrated Kt/V (eKt/V) that accounts for post-dialysis urea rebound, as urea equilibrates from tissues back into the blood compartment after treatment ends. The equilibrated value is typically 0.15–0.20 lower than single-pool.
Beyond adequacy metrics, the calculator monitors ultrafiltration rate (mL/kg/hr), an important safety parameter. Evidence from the DOPPS study shows that UF rates above 10–13 mL/kg/hr are associated with increased intradialytic hypotension, cardiac stunning, and mortality. The calculator flags higher UF-rate bands and highlights common reasons a session may score low.
Monthly Kt/V assessment is mandatory for all hemodialysis patients in the United States (CMS regulations). Accurate calculation requires the Daugirdas formula with ultrafiltration correction because simple BUN-based URR can underestimate delivered dose when significant fluid is removed. This calculator provides both metrics instantly, flags higher-risk UF-rate bands, and organizes the common low-clearance checks in one place.
URR = (PreBUN - PostBUN) / PreBUN × 100 Kt/V (Daugirdas) = -ln(R - 0.008 × t) + (4 - 3.5 × R) × UF/W where R = Post/Pre BUN ratio, t = hours, UF/W = UF volume/post-weight eKt/V = spKt/V - 0.6 × (spKt/V / t) + 0.03
Result: URR 71.4%, spKt/V 1.44
R = 20/70 = 0.286, t = 4h, UF/W = 2500/(72500) = 0.034. Kt/V = -ln(0.286 - 0.032) + (4 - 1.0) × 0.034 = 1.38 + 0.103 = 1.44. URR = (70-20)/70 × 100 = 71.4%. Both exceed KDOQI targets (URR ≥ 70%, Kt/V ≥ 1.4).
URR is simple and easy to follow, but Kt/V is the more complete dialysis-dose metric because it adjusts for ultrafiltration and treatment time. Using both together gives a better picture than relying on either measure alone.
Post-dialysis urea rebound means the immediate post-session BUN slightly overstates the delivered clearance if it is interpreted alone. eKt/V adjusts for that rebound and is especially relevant in shorter, higher-efficiency treatments.
A low URR or Kt/V does not automatically mean the machine prescription was wrong. Access recirculation, shortened treatment time, low blood flow, clotting, sampling technique, and large ultrafiltration demands can all pull the number down. The result is best used as a prompt for structured review rather than as a stand-alone judgment.
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This calculator computes URR from the pre- and post-dialysis BUN values and estimates single-pool Kt/V with the second-generation Daugirdas equation. It also shows an equilibrated Kt/V estimate and an ultrafiltration-rate check so the session can be reviewed as one adequacy worksheet rather than as a single number.
The result is intended for dialysis-dose review, not as an isolated prescription change. Sampling technique, treatment time, access performance, blood-flow limits, and the broader dialysis plan still matter when URR or Kt/V look low.
URR is a simple percentage that only measures the change in BUN. Kt/V is more comprehensive — it accounts for ultrafiltration (which concentrates urea, making removal appear less effective), treatment time, and volume of distribution. At the same URR, a patient who had 3 liters ultrafiltered will have a higher Kt/V than one with zero UF.
Equilibrated Kt/V corrects for post-dialysis urea rebound — BUN rises 10-20% within 30-60 minutes after treatment as urea equilibrates from tissues. eKt/V is the "true" delivered dose and is approximately 0.15-0.20 lower than spKt/V. It matters most for short, high-efficiency dialysis (e.g., <3 hours at high blood flow).
Excessive ultrafiltration rate (>13 mL/kg/hr) is associated with intradialytic hypotension, myocardial stunning, and increased mortality (DOPPS data). The KDOQI 2015 update recommends limiting UF rate to ≤13 mL/kg/hr. Patients requiring large fluid removal should consider longer or more frequent sessions.
Access recirculation during post-dialysis BUN sampling can falsely lower the post-BUN, inflating Kt/V. The "slow-flow" technique (reducing blood flow to 50 mL/min for 15 seconds before drawing post-BUN) prevents this. Also, lab errors, hemolyzed samples, or incorrect timing of blood draws can affect results.
KDOQI targets (≥ 1.2 minimum, 1.4 target) are based on spKt/V, which is the standard for quality reporting in the US (CMS, CROWNWeb). eKt/V is used for research and in practice to ensure the equilibrated dose is also adequate. To meet eKt/V ≥ 1.2, spKt/V usually needs to be ≥ 1.4.
Common review areas include vascular access problems, blood-flow limitations, dialyzer efficiency, dialysate settings, treatment time, and sampling technique. Access problems are among the most common causes, but the next step depends on the dialysis prescription and the broader clinical picture.