Category Archives: Basics

Transport Across Membrane

Transport of ‘toxins’, electrolytes, and water occurs from blood to dialysate and of electrolytes from dialysate to blood. This occurs via two methods: Diffusion and Convection. In this module, we will focus here only on transport of solutes from blood to dialysate.



Transmembrane Pressure


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

High Efficiency vs High Flux

High Efficiency

High Flux

 Membrane Characteristics

  •  Numerous pores, thin membrane, and high surface area allowing high small molecule (urea) clearance
  • Larger pores allowing better middle molecule (β2 microglobulin) clearance

 General Characteristics

(high diffusive clearance)

  • Dialyzer urea clearance rate (K) is usually >210 mL/min
  • High KoA (determines ‘efficiency’) – KoA urea of the dialyzer is usually > 800-1000 mL/min. Remember that the in vitro KoA provided by the manufacturer is often 20% higher than in vivo values.
  • Kuf is variable
  • Clearance of middle molecular weight molecules (β2 microglobulin) is variable
(high convective clearance)

  • Kuf (determines ‘flux’) of dialyzer is high (>15-20ml/h/mm Hg)
  • Clearance of middle molecules (β2 microglobulin ) is high (>20ml/min)

Technical Requirements

  • Dialyzer with a high KoA (>800-1000ml/min)
  • High blood flow (≥350 mL/min)
  • High dialysate flow (≥500 mL/min)
  • Bicarbonate dialysate (it is important to use  Bicarbonate-containing dialysate to avoid symptoms associated with acetate intolerance (i.e., nausea, vomiting, headache, hypotension, worsening of metabolic acidosis, and cardiac arrhythmia)
  • Dialyzer with a high Kuf
  • Ultrapure water for dialysis
  • Automated ultrafiltration control is necessary to avoid errors in TMP calculation that can result in massive flux of water across a high flux membrane and hemodynamic instability

More on High Flux Dialysis

  • HEMO study1 did not show a mortality benefit between high flux and low flux dialysis.
  • However, the HEMO study, MPO2 trial, and EGE3 trial showed a survival benefit (especially cardiovascular) in subset of patients with albumin≤4mg/dl, dialysis vintage ≥3.7 years, diabetes or those with an AV fistula.
  • KDOQI Adequacy Work group recommends use of high flux dialyzers routinely as long as appropriate water treatment is available.


  1. Eknoyan G, Beck, GJ Cheung AK, et al. Effect of dialysis dose and membrane flux in maintenance hemodialysis. N Engl J Med 2002; 347:2010. Hemodialysis study(HEMO)
  2. Locatelli, martin-Malo, Hannedouche et al. Membrane permeability Outcome (MPO) study group. Effect of membrane permeability on survival of hemodialysis patients. JASN 2009;20(3):645
  3. Asci, Tz, Ozkahya et al. EGE study group. The impact of Membrane permeability and dialysate purity on cardiovascular outcomes.JASN 2013 May;24(6):1014-23 



  • 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.



  • 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.