Daugirdas mathematical formula to calculate:
Kt/V = -ln (R – 0.03) + [(4 – 3.5R) x (UF ÷ W)]
R= post HD BUN/preHD BUN
UF= UF volume in L
W= post dialysis weight in kg
Online calculators and normograms are available to calculate Kt/V using formula above.
The quick and dirty method:
- If the dialyzer’s clearance (K) is known (based on the packet insert provide by the manufacturer at a Qb of 400ml/min) and is 250 ml/min and the dialysis session time is 240 minutes (4 hours)
- then Kt (dialysis dose)= 250 x 240 = 60,000ml or 60 liters
- If the patient weighs 70kg. His TBW is 60% of 70kg
- then V = 70 kg multiplied by .60 = 42 liters
- So the ratio – K multiplied by t to V, or Kt/V – compares the amount of water that passes through the dialyzer and is cleared of urea to the amount of water in the patient’s body.
- KT/V for this patient will be= 60/42 = 1.42
Caveat: If this same patient has 3 kg of edema fluid (EDW of 70kg) then V = 60% (70kg)+ 100%(3kg)= 45L and not 42L. This is because edema fluid adds to the TBW in its entirety as urea distributes evenly across body water.