- Choose Conventional (BUN mg/dL) or SI (urea mmol/L) units to match your lab report. The Kt/V ratio is unit-independent; switching units clears the BUN fields.
- Enter the pre-dialysis BUN (drawn from the arterial line before the pump starts) and the post-dialysis BUN (drawn within ~30 seconds of stopping, before rinse-back, using a slow-flow or stop-pump technique) from the same session.
- Enter the session duration in minutes, the ultrafiltration volume in liters (usually pre-weight minus post-weight), and the post-dialysis weight in kilograms.
- Select biological sex so total body water (V) can be estimated (Watson-style coefficient: 0.60 × weight for males, 0.55 × weight for females).
- Results update live: spKt/V, URR, estimated V, and UFR, each with a colored target badge plus a plain-language verdict and recommended action.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool to quantify the delivered dose of a single hemodialysis session. It computes single-pool Kt/V (spKt/V) by the Daugirdas second-generation equation, the urea reduction ratio (URR), an estimate of total body water (V), and the ultrafiltration rate (UFR). It is the calculation done at every monthly adequacy review and whenever a patient feels under-dialyzed.
Appropriate use
Maintenance hemodialysis patients who have a pre-dialysis BUN and a correctly timed post-dialysis BUN from the same midweek session, plus the session duration, ultrafiltration volume, post-dialysis weight, and sex (to estimate V). Most useful for confirming adequacy, troubleshooting a low Kt/V, or checking that ultrafiltration is within a safe rate.
When NOT to rely on it
The single-pool Kt/V overestimates the true (equilibrated) dose because of post-dialysis urea rebound, and it does not apply to peritoneal dialysis (PD uses a weekly Kt/V from dialysate and urine collection). The post-BUN must be drawn correctly — a sample taken after rinse-back, recirculation, or with the pump slowed will falsely inflate Kt/V. Kt/V is one dimension of adequacy; fluid status, nutrition, and symptoms matter too.
Pearls & Pitfalls
Draw the post-BUN correctly
The single biggest source of a falsely high Kt/V is a mis-timed post-dialysis sample. Use a slow-flow (≈100 mL/min for 15 s) or stop-pump technique and draw immediately at the end of treatment, before saline rinse-back, to avoid dilution and access recirculation artifacts.
Kt/V is size-normalized — V matters
Because Kt/V divides clearance by total body water, a small patient reaches target with a shorter or slower session than a large patient at identical machine settings. When Kt/V is low, increasing time, blood-flow rate (Qb), or dialyzer surface area — or addressing access recirculation — usually raises it more reliably than any single change.
Pitfalls
(1) Single-pool Kt/V overestimates the true equilibrated dose by ~0.2 due to urea rebound — a borderline spKt/V of 1.2 may be inadequate. (2) URR ignores ultrafiltration and is less precise than Kt/V; trust the Kt/V when they disagree. (3) This calculator does not apply to peritoneal dialysis, which uses a weekly Kt/V from a 24-hour dialysate and urine collection. (4) A high UFR (> 13 mL/h/kg) is a hemodynamic red flag even when Kt/V is excellent.
Why Use It
Under-dialysis is independently associated with higher mortality, worse uremic symptoms, and poorer quality of life. Kt/V expresses the cleared volume of urea (K×t) relative to the urea distribution volume (V), giving a normalized, body-size-adjusted dose that can be compared to a guideline target (spKt/V ≥ 1.4 for thrice-weekly HD). URR is a simpler, less precise companion metric. Because total body water — and therefore V — scales with weight and sex, the same machine settings deliver a very different dose in a large versus a small patient; computing V makes that explicit. A high ultrafiltration rate (UFR), meanwhile, flags hemodynamic risk independent of solute clearance.
Dialysis Adequacy Calculator — spKt/V & URR
Enter your paired BUN values and session details to calculate your spKt/V (Daugirdas 2nd generation formula), URR, estimated body water (V), and ultrafiltration rate (UFR). Find these values on your monthly dialysis lab results or in your dialysis chart.
⚕ Formula: spKt/V = −ln(R − 0.008×t) + (4 − 3.5×R) × UF/V, where R = Post-BUN/Pre-BUN, t = session time in hours. This is the Daugirdas second-generation equation — the KDIGO/KDOQI recommended standard. This calculator is for educational purposes and does not replace clinical assessment by your dialysis team.
Next Steps
Use the result to support — not replace — clinical judgment.
- Interpret the value against the targets shown in the calculator and the Evidence section below, in the context of the full clinical picture.
- Trend serial measurements rather than acting on a single result; confirm abnormal or unexpected values before changing management.
- Apply the relevant KDIGO / specialty-guideline threshold and document the indication.
- Escalate or refer to nephrology when results are out of range, rapidly changing, or discordant with the clinical picture — and discuss the implications with the patient.
Evidence & References
Formula & Equations
| Quantity | Equation |
|---|---|
| Post/pre urea ratio (R) | R = post-dialysis BUN ÷ pre-dialysis BUN |
| Single-pool Kt/V (Daugirdas 2nd generation) | spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ V, where t = session time in hours |
| Urea reduction ratio (URR, %) | URR = (pre-BUN − post-BUN) ÷ pre-BUN × 100 |
| Total body water / volume of distribution (V) | V = post-dialysis weight (kg) × 0.60 (male) or × 0.55 (female) |
| Ultrafiltration rate (UFR, mL/h/kg) | UFR = UF volume (mL) ÷ session time (h) ÷ post-dialysis weight (kg) |
Target reference ranges
| Metric | Target (thrice-weekly HD) |
|---|---|
| Single-pool Kt/V | Minimum ≥ 1.2; recommended target ≥ 1.4 (KDOQI 2015) |
| Urea reduction ratio (URR) | ≥ 65% |
| Ultrafiltration rate (UFR) | Keep < 13 mL/h/kg; ideally < 10 mL/h/kg |
The Daugirdas second-generation logarithmic equation is the KDOQI/KDIGO-endorsed bedside estimate of single-pool variable-volume Kt/V. The (4 − 3.5 × R) × UF/V term accounts for convective urea removal during ultrafiltration. Single-pool Kt/V overestimates the equilibrated Kt/V by roughly 0.2 because of urea rebound; UFR thresholds derive from observational dialysis-cohort data.
Evidence & References
The spKt/V equation is the Daugirdas second-generation formula (1993), adopted as the standard bedside method by KDOQI and KDIGO. The minimum spKt/V ≥ 1.2 and target ≥ 1.4 for thrice-weekly hemodialysis come from the KDOQI 2015 Hemodialysis Adequacy update, informed by the HEMO Study, which found no survival benefit from a dose above this threshold. URR ≥ 65% is the corresponding historical CMS/KDOQI benchmark.
- Daugirdas JT. Second generation logarithmic estimates of single-pool variable volume Kt/V: an analysis of error. J Am Soc Nephrol. 1993;4(5):1205–1213.
- National Kidney Foundation. KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 Update. Am J Kidney Dis. 2015;66(5):884–930.
- Eknoyan G, Beck GJ, Cheung AK, et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis (the HEMO Study). N Engl J Med. 2002;347(25):2010–2019.
- Watson PE, Watson ID, Batt RD. Total body water volumes for adult males and females estimated from simple anthropometric measurements. Am J Clin Nutr. 1980;33(1):27–39.
- Flythe JE, Kimmel SE, Brunelli SM. Rapid fluid removal during dialysis is associated with cardiovascular morbidity and mortality. Kidney Int. 2011;79(2):250–257.
