How this calculator works
This reproduces the 1993 second-generation variable-volume single-pool equation. It uses session hours, removed fluid in litres and post-dialysis weight in kilograms. The 0.008 urea-generation term belongs to the conventional schedule; different dialysis frequencies may need another model. This is neither equilibrated nor weekly standard Kt/V. Concentration inverse modes solve the original monotonic equation numerically. The result is an estimate for information only and is not medical advice.
Formula
R = post-dialysis / pre-dialysis BUN. spKt/V = -ln(R - 0.008t) + (4 - 3.5R) × UF/W, with t in hours, UF in litres and W in kg. URR = 1 - R. The logarithm requires R > 0.008t. Duration and fluid/weight inverses rearrange this same equation; concentration inverses use bisection on 0.008t < R ≤ 1.
Worked example
With R = 0.5, 4 hours and zero ultrafiltration, Kt/V = -ln(0.468) = 0.75928698. With the same R, time and post-weight 50 kg, removing 2 L adds (4 - 1.75) × 2/50 = 0.09, giving 0.84928698.
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Frequently asked questions
Why is Kt/V not just minus log of post/pre?
The Daugirdas equation also accounts approximately for urea generation and changing fluid volume.
Can I use this for daily dialysis?
The original fixed generation factor may be inaccurate for other frequencies. A dialysis team should use a validated schedule-specific approach.
Why can a known Kt/V fail to give a solution?
Some combinations imply negative fluid removal, non-positive duration or an impossible post/pre ratio. The calculator rejects those combinations.