Case 2: Constrained T

Not a minute more than 3.5 hours!

67 year old African American male with history of diabetes, hypertension and advanced chronic kidney disease has been initiated on dialysis in the hospital. He has received 6 treatments so far and has done well without significant issues with hypotension or cramping. He will now be transitioning to your outpatient dialysis unit. The patient weighs 70kg and has a well functioning AV fistula. You have done your smart calculations (Case 1) and know that he needs to be dialyzed for 3.9 hours for a goal sp Kt/V of 1.4. However, the patient is adamant that he will not stay for more than 3.5 hours because that’s what his friend does. How would you calculate the initial dialysis prescription now?

STEP 1: Start with T since it is a non-negotiable for the patient
T = 3.5 hours or 210 min

STEP 2: Calculate your V
V = 60% of 70kg = 42L or 42,000 ml

STEP 3: Remember your goal spKt/V
spKt/V = 1.4

STEP 4: Plug in the known variables for Kt/V
[K x 210min] / 42,000ml = 1.4
K = [42,000ml x 1.4] / 210min = 280ml/min

STEP 5: Assume that you would like to prescribe a Qb of 400ml/min and a Qd of 500ml/min

STEP 6: Refer to any normogram or calculators easily available online to decide what your dialyzer’s KoA should be in order to achieve a spKt/V of 1.4

STEP 7: Check the normogram to make sure the in vitro KoA of the dialyzer should at least be 766ml/min.


  • As mentioned in the earlier case, in vitro KoA values (provided by manufacturer packet insert) are about 20% higher than in vivo values. Therefore when using in vitro K and KoA values to calculate dialysis prescription, it is important to confirm delivered spKt/V and adjust treatment as needed to meet goal.