Kt/V Calculator (Dialysis Adequacy)
Kt/V Calculator (Dialysis Adequacy)
Calculate delivered Kt/V with the Daugirdas second-generation single-pool formula, the standard measure of haemodialysis adequacy.
Kt/V (Dialysis Adequacy)
5 inputs → Kt/VPre-dialysis BUN 60 mg/dL, post-dialysis BUN 18 mg/dL, session 4 hours, ultrafiltration 2.5 L, post-dialysis weight 70 kg
Formula
- R
- post-dialysis BUN ÷ pre-dialysis BUN
- t
- dialysis session length in hours
- UF
- ultrafiltration volume in litres removed during the session
- W
- post-dialysis weight in kg
Worked example
Pre-dialysis BUN 60 mg/dL, post-dialysis BUN 18 mg/dL, session 4 hours, ultrafiltration 2.5 L, post-dialysis weight 70 kg
R = 18 ÷ 60 = 0.300
R − 0.008 × 4 = 0.300 − 0.032 = 0.268
−ln(0.268) = 1.317
(4 − 3.5 × 0.300) × 2.5 ÷ 70 = 2.95 × 2.5 ÷ 70 = 0.105
1.317 + 0.105 = 1.42
Between 1.2 and 1.8 → adequate
KDOQI targets
| Kt/V | Adequacy | Action |
|---|---|---|
| < 1.2 | Below target | Review recirculation, prescribed time, blood and dialysate flow |
| 1.2 – 1.8 | Adequate | Meets the KDOQI minimum spKt/V of 1.2; target ≥ 1.4 to allow for shortfalls |
| > 1.8 | High | Verify sampling technique before treating the dose as genuinely high |
Sampling technique decides the result
Kt/V is the ratio of urea clearance to urea distribution volume delivered in a session, and it has replaced simpler measures because it accounts for treatment time and ultrafiltration alongside the urea reduction itself. The Daugirdas second-generation formula is a logarithmic approximation of the underlying kinetic model that avoids needing to solve the variable-volume differential equation directly, and it is accurate enough for routine practice.
The single most important source of error is not the arithmetic but the post-dialysis blood sample. Urea rebounds for 30 to 60 minutes after a session ends as it re-equilibrates from tissue compartments into the blood, so drawing the sample too soon underestimates post-dialysis urea and produces a falsely high Kt/V. Access recirculation causes the same distortion during the session itself, diluting the sampled blood with already-cleared blood and understating true urea. Both push the number in the same direction, which is why an unexpectedly high Kt/V should prompt a check of technique before a change in prescription.
KDOQI sets 1.2 as the minimum delivered spKt/V for thrice-weekly haemodialysis, with a working target of 1.4 to leave margin for missed or shortened sessions. Residual native kidney function reduces the dialysis dose needed to reach the same overall clearance, and should be factored in separately when it is still measurable. Peritoneal dialysis uses a different weekly target and a different formula entirely.
Frequently asked questions
What is a good Kt/V for dialysis?
KDOQI sets 1.2 as the minimum delivered single-pool Kt/V for thrice-weekly haemodialysis, with a target around 1.4 to allow for occasional shortened sessions.
Why does Kt/V read falsely high?
Most often because the post-dialysis sample was drawn too soon, before urea rebound from tissue compartments has occurred, or because of access recirculation diluting the sample during dialysis. Both underestimate true post-dialysis urea and inflate the calculated Kt/V.
How should the post-dialysis sample be taken?
With a slow-flow or stop-dialysate technique that avoids drawing recirculated blood from the access, and ideally accounting for the rebound period rather than sampling immediately at line disconnection.
Does Kt/V apply to peritoneal dialysis?
No — peritoneal dialysis adequacy uses a weekly Kt/V with a different target and a different calculation, reflecting continuous rather than intermittent clearance.
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References
- Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error. J Am Soc Nephrol. 1993;4(5):1205–13.
- National Kidney Foundation. KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 Update. Am J Kidney Dis. 2015;66(5):884–930.