This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Measuring the dose of dialysis
Hemodialysis replaces part of the work of failed kidneys by filtering the blood, usually three times a week for about four hours. Doctors need a way to check that each session removes enough waste. Urea, a small molecule produced when the body breaks down protein, is easy to measure and serves as the marker. Kt/V and the urea reduction ratio (URR) both come from blood urea levels drawn before and after a session.
The urea reduction ratio
URR = (1 − post urea ÷ pre urea) × 100
A pre-dialysis BUN of 60 mg/dL falling to 18 mg/dL gives URR = (1 − 0.30) × 100 = 70%. The usual minimum is 65%.
The Daugirdas formula for Kt/V
spKt/V = −ln(R − 0.008 × t) + (4 − 3.5 × R) × UF ÷ W
- R is the ratio of post- to pre-dialysis urea.
- t is the session length in hours.
- UF is the fluid removed in liters, and W the post-dialysis weight in kg.
The 0.008 × t term accounts for urea produced during the session; the second term adds the urea removed with the extra fluid.
Example
With R = 0.30, a 4-hour session, 2.5 L removed and a post-dialysis weight of 154 lb (69.9 kg):
- −ln(0.30 − 0.032) = 1.317
- (4 − 1.05) × 2.5 ÷ 69.9 = 0.106
- spKt/V ≈ 1.42, which meets the target of 1.4.
Equilibrated Kt/V
After the session, urea flows back from the tissues into the blood for 30 to 60 minutes, so the single-pool value overestimates the dose. The Tattersall-Daugirdas rate equation, eKt/V = spKt/V − 0.6 × spKt/V ÷ t + 0.03, estimates the value after this rebound for a fistula or graft. In the example it is about 1.24.
When the numbers fall short
A low Kt/V can come from a session that was shortened, a low blood flow, access recirculation, a clotted dialyzer or a sampling error. The care team usually repeats the measurement and then adjusts the session length, the blood flow or the dialyzer. Never change a prescription on your own.