Kt/V Dialysis Adequacy Calculator (Daugirdas Formula)
Kt/V is the standard measure of hemodialysis adequacy, quantifying the clearance of urea during a dialysis session. This calculator uses the Daugirdas formula for single-pool Kt/V (spKt/V) estimation.
About
Kt/V is a dimensionless number that represents the fractional clearance of urea during hemodialysis, where K is dialyzer clearance of urea (mL/min), t is dialysis time (min), and V is the volume of distribution of urea (approximately total body water in mL). The Daugirdas second-generation formula, published in 1993, is the most widely used method for calculating single-pool Kt/V from pre- and post-dialysis BUN measurements. It calculates Kt/V = -ln(R - 0.008 × t) + (4 - 3.5 × R) × UF/W, where R = post-BUN/pre-BUN, t = session length in hours, UF = ultrafiltrate volume in liters, and W = post-dialysis weight in kg. The National Kidney Foundation's KDOQI guidelines recommend a minimum spKt/V of 1.2 per session for thrice-weekly hemodialysis. For twice-weekly hemodialysis, a minimum Kt/V of 1.7 is recommended. The NKF-KDOQI target is spKt/V ≥1.4 to ensure the minimum is consistently achieved. The Kt/V should be measured at least monthly in stable patients and more frequently in unstable patients or after prescription changes.
Formula
Kt/V = -ln(R - 0.008 × t) + (4 - 3.5 × R) × UF/W
Where R = post-dialysis BUN / pre-dialysis BUN, t = dialysis session length in hours, UF = ultrafiltrate volume in liters (fluid removed), W = post-dialysis weight in kg. The Daugirdas formula accounts for urea generation during dialysis and ultrafiltration to provide an accurate estimate of single-pool Kt/V.
Score Interpretation
Kt/V is the gold standard measure of hemodialysis adequacy and is strongly associated with patient outcomes. Multiple observational studies have demonstrated that higher Kt/V is associated with reduced mortality in hemodialysis patients. The landmark HEMO study (2002) showed that achieving a higher dose of dialysis (eKt/V 1.53 vs 1.16) did not significantly alter mortality in the overall cohort but showed benefits in certain subgroups. The KDOQI guidelines mandate monthly Kt/V monitoring with a minimum spKt/V of 1.2 per session. Inadequate Kt/V is associated with increased morbidity, hospitalization, and mortality, emphasizing the clinical importance of regular adequacy assessment.
Adequate (2x/week) — 1.7+
Kt/V ≥1.7. Adequate for both twice-weekly and thrice-weekly HD.
Management: Continue current prescription. Monitor monthly.
Adequate (3x/week) — 1.2–1.69
Kt/V 1.2-1.69. Adequate for thrice-weekly HD.
Management: Continue current prescription. Attempt to increase towards KDOQI target of ≥1.4.
Inadequate — 0–1.19
Kt/V <1.2. Below minimum KDOQI adequacy threshold.
Management: Increase dialysis time, blood flow rate, or dialyzer size. Optimize vascular access. Recheck after adjustment.
Reference Ranges
| Population | Normal Range | Notes |
|---|---|---|
| Thrice-weekly hemodialysis | ≥1.2 | KDOQI minimum; target spKt/V ≥1.4 |
| Twice-weekly hemodialysis | ≥1.7 | Minimum for twice-weekly schedule |
Dr. Khaled Hassan
Dr. Khaled is a cardiology consultant with experience in acute cardiac care.
View medical review board & editorial policy →Example Calculation
A 62-year-old man on thrice-weekly hemodialysis: pre-BUN 78 mg/dL, post-BUN 24 mg/dL, session length 4 hours, UF removed 2.5 L, post-dialysis weight 72 kg. R = 24/78 = 0.308. Kt/V = -ln(0.308 - 0.008 × 4) + (4 - 3.5 × 0.308) × 2.5/72 = -ln(0.276) + (4 - 1.078) × 0.0347 = 1.287 + 2.922 × 0.0347 = 1.287 + 0.101 = 1.39. Result: Kt/V 1.39 — Adequate for thrice-weekly HD and meets KDOQI target.
Related Conditions
Related Medications
Common Mistakes
Using the single-pool (spKt/V) when equilibrated (eKt/V) is required
The Daugirdas formula gives single-pool Kt/V. To estimate equilibrated Kt/V (accounting for urea rebound), subtract 0.6 × (K/V) × t where K/V is approximated. Most clinical guidelines cite spKt/V targets.
Drawing post-dialysis BUN too early or too late
Post-dialysis BUN should be drawn using the slow-flow method (decrease blood flow to 50-100 mL/min for 10-15 seconds) or stop-pump method (stop dialysate flow before drawing). Drawing immediately without slowing flow or after a delay will give inaccurate results.
Frequently Asked Questions
What is the difference between spKt/V, eKt/V, and URR?
How often should Kt/V be measured?
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):1205-1213. PubMed
- Eknoyan G, Beck GJ, Cheung AK, et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis (HEMO Study). N Engl J Med. 2002;347(25):2010-2019. PubMed
- NKF KDOQI Clinical Practice Guideline for Hemodialysis Adequacy: 2015 Update. Am J Kidney Dis. 2015;66(5):884-930. PubMed