Kt/V Calculator (Daugirdas second-generation formula, hemodialysis adequacy)

Understanding the Kt/V calculator (Daugirdas) — concept and principle

Kt/V is the standard measure of how much dialysis a patient receives in one haemodialysis session. This calculator uses the second-generation Daugirdas formula to estimate single-pool Kt/V from blood urea before and after dialysis, session length, fluid removed and weight. It also shows the urea reduction ratio (URR).

Urea before and after dialysis: with the default inputs the result is spKt/V 1.37 (meets 1.2 · URR 68.6%).

How it is calculated

R = post-dialysis BUN ÷ pre-dialysis BUN

spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W

URR (%) = (1 − R) × 100

t is session length in hours, UF is fluid removed in litres and W is post-dialysis weight in kg.
Any urea unit works as long as both samples use the same one.

The principle

K is the dialyser's urea clearance, t the time and V the volume of body water in which urea is distributed. Kt/V compares the volume of blood cleared with that body water. If urea fell exponentially and nothing else happened, Kt/V would equal −ln R. The 0.008 × t term corrects for urea produced by the body during the session, and the ultrafiltration term adds the urea removed with the extra fluid as the body water shrinks.

Worked example

Default inputs: pre BUN 70, post BUN 22, 4 hours, UF 2.5 L, weight 70 kg.

R = 22 ÷ 70 = 0.314.
−ln(0.3143 − 0.032) = −ln(0.2823) = 1.265.
(4 − 3.5 × 0.3143) × 2.5 ÷ 70 = 2.900 × 0.0357 = 0.104.
spKt/V = 1.265 + 0.104 = 1.37 → meets the 1.2 minimum: Yes.
URR = (1 − 0.3143) × 100 = 68.6%.

Each point is the calculator re-run with only “Post-dialysis BUN (mg/dL)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Post-dialysis BUN (mg/dL)	Single-pool Kt/V (spKt/V)
5.5	3.2
11	2.2
16.5	1.7
22	1.37
27.5	1.11
33	0.91
44	0.58

How to read the result

Where does your result fall?

For three sessions a week, KDOQI recommends a target spKt/V of 1.4 per session and a minimum delivered dose of 1.2. A URR of about 65% or more corresponds roughly to the minimum. Kt/V is one measure of adequacy; symptoms, fluid control, nutrition and blood tests matter too.

Common mistakes

Drawing the post-dialysis sample immediately without slowing the blood pump, or long after the end, which distorts R.
Using pre-dialysis weight.
Comparing spKt/V with equilibrated or standard Kt/V targets.

Keep in mind: This is a calculation aid, not medical advice. Dialysis prescriptions are set by the nephrology team.

Frequently asked questions

What is a good Kt/V for haemodialysis?

For thrice-weekly dialysis, a target of 1.4 with a minimum of 1.2 per session.

What is the difference between Kt/V and URR?

URR uses only the urea fall; Kt/V also accounts for urea generation and fluid removal.

How can Kt/V be increased?

Longer sessions, higher blood flow or a larger dialyser are common options, decided by the care team.

Sources: 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

Checked 9 Nov 2026. 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 (standard formula); targets spKt/V 1.4 and minimum 1.2 per thrice-weekly session as summarised by the Yale Hemodialysis Kinetics resource (KDOQI hemodialysis adequacy guideline).

Your numbers