What is Kt/V in Dialysis?
The Kt/V is the most widely used index to measure dialysis adequacy and efficiency. It describes how effectively a dialyzer removes urea from a patient's blood during a single treatment session.
The abbreviation stands for:
- K — dialyzer clearance (the rate of blood passage through the dialyzer, in mL/min)
- t — treatment duration (time of the dialysis session, in hours)
- V — volume of distribution (total body water, in liters)
Kt/V is a dimensionless number — it has no units and represents the fraction of urea-containing water cleared during one session. A Kt/V of 1.4 means that a volume of urea-containing fluid equal to 1.4 times the total body water was cleared.
What is Normal Kt/V?
Clinical guidelines define the following adequacy thresholds for hemodialysis:
- Kt/V ≥ 1.4 — Optimal. KDOQI recommends this as the delivered dose target.
- Kt/V ≥ 1.2 — Adequate minimum. Dialysis must not fall below this value.
- Kt/V < 1.2 — Inadequate. Treatment review and prescription adjustment required.
For peritoneal dialysis, the target is a weekly Kt/V ≥ 1.7 per the International Society of Peritoneal Dialysis (ISPD). In Acute Kidney Injury (AKI), current guidelines recommend a weekly Kt/V of at least 3.9.
Kt/V vs URR — What Is the Difference?
The Urea Reduction Ratio (URR) is a simpler measure of dialysis adequacy:
URR (%) = (Pre-BUN − Post-BUN) / Pre-BUN × 100
A URR ≥ 65% is considered adequate. However, URR does not account for convective urea removal (through ultrafiltration) or urea rebound, making Kt/V — particularly the Daugirdas formula — a more comprehensive and clinically preferred measure.
How to Calculate Kt/V (Daugirdas Formula)
This calculator uses the Daugirdas second-generation single-pool Kt/V formula, which is the clinical standard recommended by KDOQI:
Kt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF / W
Where:
- R = post-BUN / pre-BUN (urea ratio)
- t = session duration in hours
- UF = ultrafiltration volume in liters
- W = post-dialysis body weight in kg
- ln = natural logarithm
American system: enter BUN in mg/dL and weight in lbs — the calculator converts lbs to kg
automatically (1 lb = 0.4536 kg).
Metric system: enter BUN in mmol/L and weight in kg directly.
Since Kt/V uses the ratio R = post-BUN/pre-BUN, the formula gives identical results for mg/dL and mmol/L inputs — only the displayed unit label changes.
FAQs
Why does Kt/V disadvantage women and smaller patients?
Women and smaller individuals have proportionally less total body water (V) relative to their lean body mass. A lower V produces a higher calculated Kt/V for the same dialysis dose, which can give a misleading picture of adequacy. Clinical guidelines therefore recommend targeting the upper end of Kt/V targets for these patient groups.
What BUN values should I enter?
Enter the pre-dialysis BUN (measured immediately before the session starts) and the post-dialysis BUN (measured 30–60 seconds after blood flow stops, before the patient disconnects). Using the equilibrated post-BUN (measured 30 minutes after the session) gives a more accurate single-pool Kt/V.
What ultrafiltration volume should I use?
Enter the total fluid removed during the session in liters. This is typically recorded by the dialysis machine as the ultrafiltration goal achieved.
Can I use this for peritoneal dialysis?
This calculator is optimized for hemodialysis (Daugirdas formula). For peritoneal dialysis, Kt/V is calculated differently — using the weekly urea clearance and residual kidney function. The target weekly Kt/V for peritoneal dialysis is ≥ 1.7 (ISPD guidelines).
⚠️ This calculator is for informational and clinical reference purposes only. It does not replace a physician's clinical assessment. Always consult a qualified nephrologist for diagnosis and treatment decisions.