Clinical Pearls



  • During conventional intermittent hemodialysis, urea is mostly cleared by diffusion given its small solute size and high concentration gradient between blood and dialysate.
  • Drugs that have a molecular weight of <1000 Da have high water solubility and low protein binding in plasma are cleared well with hemodialysis. Lithium is one such drug. Hemodialysis is the treatment of choice for severe lithium toxicity.



  • During conventional intermittent hemodialysis, urea is cleared mostly by diffusion. However, convection contributes in small amounts to urea clearance. Higher the ultrafiltration during a given treatment, more the convective clearance of urea.
  • With the use of high flux dialyzers in the current era and better middle molecule clearance, the risk of β2 microglobulin associated amyloidosis in hemodialysis patients is becoming a rarer entity.

Transmembrane Pressure (TMP)


In modern dialyzers with volumetric control of ultrafiltration, TMP’s primary role is to help monitor filter function:
↓   A drop in TMP could be due to a leak or filter rupture
↑  A rise in TMP could be due to filter clotting

What is the Goal Kt/V?


  • Based on large retrospective studies, a Kt/V <1.0 is associated with poor outcomes. Clinical practice guidelines, therefore, recommend a minimum delivered spKt/V of 1.2.
  • Delivered Kt/V is often lower than the prescribed Kt/V. This could be related to multiple reasons such as interrupted treatment due to machine alarms, access recirculation, blood or dialysate pump calibration errors that leads to decreased delivered Qb or Qd etc.
  • Therefore, to achieve a delivered spKt/V of 1.2, a target spKt/V of 1.4 is recommended. This equates to an eKt/V of 1.2

High Efficiency vs Conventional Dialyzers


  • Most dialyzers that are currently in use have a KoA between 800-1600ml/min.
  • In order to get maximum benefit from a high efficiency dialyzer, you need adequate Qb.

K increases with increase in Qb


  • Dialyzer blood flow rate is a big determinant of K and ultimately Kt/V. Ideally Qb should be set between 400ml/min to 500ml/min as tolerated. Therefore when aiming for a goal Kt/V , ensure the delivered Qb is adequate.
  • Dialysis catheters tend to allow lower Qb than AVF and AVG.
  • K can NEVER exceed the Qb. If all the blood entering the dialyzer is cleared of urea in a minute, K is Qb.

K increases with increase in Qd, but…


  • Higher Qd increases dialyzer urea clearance (K) but as Qb approaches Qd, K starts to plateau. Therefore,  Qd should typically be 1.5-2 times the Qb to maximize diffusive clearance.
  • At a Qb of 400-500 ml/min, a Qd of UP TO 800ml/min makes sense.
  • An increase of Qd from 500ml/min to 800ml/min increases K only by 8-12% when the Qb and KoA are not limiting i.e. a high efficiency dialyzer is used and the Qb >400ml/min (as there is more effective surface area for diffusion) .
  • Increasing Qd >800ml/min usually doesn’t add to the K or KT/V as the Qb becomes limiting (shown in graph). Dialystate is expensive, why waste it?
  • Qd of 500-600ml/min is often adequate to achieve target urea clearance with a conventional Qb of 400ml/min
  • An additional lesson is that with daily hemodialysis methodologies that have reduced Qds of 150 mL/min (for example, NxStage) or continuous veno-venous hemodialysis (CVVHD) techniques with Qds of 50–100 mL/min, there is no reason to employ higher Qbs or to use large dialyzers, as K will be limited by Qd.

K increases with increase in KoA


  • K increases with increasing dialyzer efficiency (KoA)
  • However, the increase in K is most pronounced at adequate pump blood flow.
  • At a Qb of <200ml/min, despite using a high efficiency dialyzer, K is low (the curves start to converge at a K of <170ml/min as shown in the graph).
  • At a Qb of >200ml/min, the curves start to separate and K rises in proportion to the rise in KoA. Rise in K at this point is therefore ‘membrane limited’. Hence, if you want the most benefit out of your high efficiency dialyzer, you need adequate blood flow preferably >400ml/min

Case 1: Dialysis Prescription 


  • The delivered Kt/V often tends to be lower than the prescribed Kt/V which is why we target a spKt/V of 1.4 to achieve a minimum goal spKt/v of 1.2.
  • When calculating treatment time using manufacturer’s provided in vitro K and KoA values (as we did in this case), it is important to remember that these values tend to be about 20% higher than actual in vivo values. This is one of the reasons why delivered Kt/V may be lower than prescribed Kt/V.
  • In smaller patients, determining treatment time using goal spKt/V of 1.4 may lead you to under dose dialysis since a low ‘V’, mathematically increases the ratio spKt/V, gives a false sense of reassurance and may prompt you to decrease dialysis time. Smaller patients have higher post dialysis urea rebound, so their eKt/V may be significantly lower than spKt/V. In addition, short dialysis treatments may not be adequate to remove middle molecules and other uremic solutes as well as may result in higher ultrafiltration rates and intra-dialytic hypotension. Therefore before reducing dialysis time, other variables such as BP, phosphorus and potassium control, ultrafiltration goals etc. need to be considered. US KDOQI 2006 recommends a minimum session length of 3 hours irrespective of Kt/V.

Case 2: Constrained T


  • 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.

Access Recirculation


  • The higher the blood pump flow rate, the higher the chances of access recirculation.
  • 25% of patients with a drop in Kt/V tend to have access recirculation issues.