Kt/V Calculator (Hemodialysis Adequacy)
Calculate single-pool Kt/V with the Daugirdas second-generation formula, the urea reduction ratio (URR) and equilibrated Kt/V from pre- and post-dialysis urea, session time, ultrafiltration and weight.
How it works
spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W (Daugirdas 1993) · R = post ÷ pre urea, t = hours, UF = litres removed, W = post-dialysis weight (kg) · URR = (1 − R) × 100% · eKt/V = spKt/V − 0.6 × spKt/V ÷ t + 0.03
Kt/V is the dialyzer urea clearance (K) multiplied by treatment time (t) and divided by the patient’s urea distribution volume (V) — the dose of dialysis expressed as how many times the body water is cleared of urea. Daugirdas’ second-generation formula estimates single-pool Kt/V from the fall in urea during the session, adding a term for the urea removed with ultrafiltered fluid. KDOQI (2015) recommends a target spKt/V of 1.4 per session for thrice-weekly hemodialysis, with a minimum delivered dose of 1.2; the URR (minimum about 65%) is a simpler but less complete measure. Equilibrated Kt/V accounts for urea rebound after the session.
Worked example
Pre-dialysis urea 25 mmol/L, post-dialysis 8 mmol/L, a 4-hour session removing 2 L, post weight 70 kg: R = 0.32, spKt/V = −ln(0.32 − 0.032) + (4 − 1.12) × 2 ÷ 70 = 1.245 + 0.082 = 1.33 — above the 1.2 minimum but below the 1.4 target. URR = 68.0% and eKt/V = 1.16.
Sources: Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error. J Am Soc Nephrol 1993;4:1205–13 · National Kidney Foundation. KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 update. Am J Kidney Dis 2015;66:884–930
Frequently asked questions
What is a good Kt/V for hemodialysis?
For thrice-weekly hemodialysis, KDOQI 2015 recommends a target single-pool Kt/V of 1.4 per session and a minimum delivered spKt/V of 1.2. Peritoneal dialysis uses a weekly Kt/V target of at least 1.7.
What is the difference between Kt/V and URR?
URR is just the percentage fall in urea (target 65–70% or more). Kt/V also accounts for urea generated during the session and removed with ultrafiltration, so two patients with the same URR can have different Kt/V. Roughly, URR 65% ≈ spKt/V 1.2 and URR 70% ≈ 1.4.
How should the post-dialysis sample be drawn?
Use a slow-flow or stop-pump technique at the end of the session (blood pump slowed to about 100 mL/min for 15 seconds) to avoid dilution by access recirculation, which would make the post value too low and the Kt/V falsely high.
Can I use BUN in mg/dL instead of urea in mmol/L?
Yes. Only the ratio of post to pre values enters the formula, so any unit works as long as both samples use the same one.
Related calculators
Educational calculation only, not medical advice or a diagnosis. Reference ranges and units differ between laboratories; interpret any result with your doctor in the context of your full clinical picture.