Definitions of CRRT terminology

This has been asked about in Question 30 from the second paper of 2014 and Question 14 from the second paper of 2007. The best resource for such definitions was previously the ADQI site, which (in the "ADQI Reports" section under "CRRT") had a "Definitions/nomenclature" workgroup statement. The definitions in this statement are used wherever possible in the following text. Unfortunately the site was restructured and the definitions disappeared, but the nomenclature still exists, and is described in a 2016 consensus statement.


Diffusion is the transport of solute across a membrane, along a concentration gradient. This is how small molecules are cleared during haemodialysis. The physicist/chemist definition is "Diffusion is the net movement of a substance from a region of high concentration to a region of low concentration."

The ADQI definition is:

Diffusion describes solute transport across a semi-permeable membrane generated by a concentration gradient.


Convection is the transport of a solute across a membrane along with solvent (by "solvent drag"). The physicist/chemist definition is "Convection is the collective movement of molecules within fluids."

The ADQI definition is:

Convection is bulk-flow of solute across a semi-permeable membrane together with a solvent in a manner that is dependent on transmembrane pressure and membrane characteristics.

This is how small and middle molecules are cleared during haemofiltration.

Dialysis dose


  • Definition of "dose" in CRRT is volume of blood "purified."
  • Measure of "dose" in CRRT: clearance rate of a representative marker solute.


  • Dialysis dose is equivalent to the effluent rate in ml/kg/hour.
  • Effluent rate is the ultrafiltration rate for haemofiltration (CVVH), or the sum of ultrafiltration rate and dialysis rate for CVVHDF

Relevance to CRRT:

  • Prescribed dose is the effluent rate
  • No benefit in outcomes with doses in excess of 25ml/kg/hr
  • Practically, higher doses may be necessary to compensate for circuit downtime

Traditionally, urea has been the marker solute since the clearance of all accumulated "renal toxins" correlates to some extent with urea clearance. However, it is not used to prescribe a dialysis dose.

Most studies prescribe weight-based effluent rates as their dose of dialysis. In practical terms, the "volume of blood purified" dose of dialysis is essentially the effluent flow rate (i.e. the combination of dialysate and ultrafiltrate flow rates).Thus, when one signs the patient up for "two litre exchanges" one is prescribing an effluent rate of 2000ml/hr, and a replacement rate of 2000ml/hr (that is the "exchange volume"). This works out to be a dialysis dose of 2000ml/hr, or 28.6ml/kg/hr for a 70kg lump of human tissue. This prescribed "dose of effluent" is usually about 20% less than the actual dose of dialysis measured by marker solute, largely because the filter efficiency tends to diminish over the course of a CRRT session.

There is some disagreement as to what the ideal dialysis dose should be.

Thus far, nobody has demonstrated any improvement in outcomes with doses higher than 25ml/kg/hr. Practically speaking, one may wish to prescribe a higher dose to compensate for anticipated filter downtime.

Transmembrane pressure

The ADQI definition is:

Transmembrane pressure is the hydrostatic pressure gradient across the membrane. This is the driving force that causes ultrafiltration.

The Transmembrane Pressure Formula:

Relevance to CRRT:

  • High TMP with normal return pressure suggests there is a filter problem
  • High TMP with high return pressure suggests either the filter or the return line are the problem
  • At any given TMP, the actual ultrafiltration rate will vary for membranes with different permeabilities

Transmembrane pressure is the pressure gradient across the filter membrane, i.e. the difference in pressure between the blood compartment and the dialysate compartment.

The college examiners give this as their definition:

Transmembrane pressure (TMP) = (Filter pressure + Return pressure) / 2 – (Effluent pressure)

Thus, TMP is the effluent pressure subtracted from the average of the pressures in the blood side of the circuit (which are the filter pressure and the return pressure).

Sieving coefficient

Sieving coefficient is the ratio of a solute in the ultrafiltrate in comparison to its concentration in the plasma which returns to the patient. The correct phrase is "a measure of equilibration between the concentrations of two mass transfer streams". The sieving coefficient describes the efficiency of solute removal by ultrafiltration, and depends upon the properties of the membrane and on the rate of ultrafiltration.


Sieving coefficient (SC) = Ultrafiltrate concentration / Blood concentration

Relevance to CRRT:

  • The sieving coefficient of any given molecule determines the success of its removal by CVVHDF

A sieving coefficient is the measure of how easily a substance passes from the blood compartment to the dialysate compartment in a filter.

Verbatim from the college model answer:

SC is a measure of how effectively a substance is removed through the filter.

SC = 0 means the substance is not filtered at all e.g. protein sized molecules

SC = 1 means the substance is completely filtered e.g. urea, creatinine

SC depends on numerous factors, and is different for each molecule.

Some of its determinants include:

  • Molecule size of the solute
  • Protein binding of the solute
  • Charge of the solute and of the filter membrane
  • Size and number of pores in the filter membrane

Filtration fraction

The filtration fraction is the volume of plasma removed from the dialysed blood by ultrafiltration. The correct phrase is "the ratio of ultrafiltration rate to plasma water flow rate".


Filtration fraction = Ultrafiltration rate / Blood flow rate

more accurately:

Filtration fraction = Ultrafiltration rate / blood pump rate × (1 – Haematocrit)

Relevance to CRRT:

  • The filtration fraction is literally the fraction of plasma which is removed from blood during haemofiltration
  • The ideal filtration fraction at a haematocrit of 0.30 is around 0.25
  • Anything higher than this increases the risk of filter failure due to haemoconcentration.
  • A higher TMP increases the filtration fraction
  • A higher blood oncotic pressure decreases the filtration fraction

An "ideal" filtration fraction is one which maximises the fluid removal while minimising the effects of the increased haematocrit on the filter lifespan (or, conversely, the effect of the filter on the patient's lifespan - as haemolysis may occur, not to mention loss of blood into the circuit).

Now; the full unabridged list of ADQI definitions:

Adsorption: An extracorporeal purification process where solute in plasma or blood binds to membranes or substances such as charcoal, resins, gels, proteins or monoclonal antibodies.

Continuous renal replacement therapy (CRRT) is any extracorporeal blood purification therapy intended to substitute for impaired renal function over an extended period of time and applied for, or aimed at being applied for, 24 hours per day.

Convection: Bulk-flow of solute across a semi-permeable membrane together with a solvent in a manner that is dependent on transmembrane pressure and membrane characteristics.

Dialysate: A solution of variable composition designed to facilitate diffusion of solutes into the ultrafiltrate-dialysate compartment of the hemofilter or hemodialyzer.

Diffusion: Describes solute transport across a semi-permeable membrane generated by a concentration gradient.

Hemodiafiltration (HDF): A technique associated with high ultrafiltration rates and diffusion across a highly permeable membrane. Blood and dialysate are circulated as in hemodialysis, but in addition, ultrafiltration, in excess of the scheduled weight loss, is provided. Replacement fluid is used to achieve fluid balance.

Hemodialysis (HD): An extracorporeal, primarily diffusive therapy, where solute and water are transported across a semi-permeable membrane into dialysate.

Hemofiltration (HF): An extracorporeal, primarily convective therapy, where solute and water are transferred across a semi-permeable membrane. Replacement fluid is used to achieve fluid balance.

High flux: A dialysis membrane designed to provide high water permeability, thereby increasing solute clearance especially large solutes such as beta-2 microglobulin.

Intermittent therapies are those usually prescribed for a period of 12 hours or less. These include Extended Daily Dialysis (EDD) and Slow Low-Efficiency Dialysis (SLED).

Peritoneal dialysis: An intracorporeal therapy where solute and water are transported across the peritoneal membrane based on osmotic and concentration gradients.

Postdilution fluid: is infused distal to the hemofilter/dialyzer.

Predilution fluid: is infused proximal to the hemofilter/dialyzer.

Replacement (substitution) fluid: A solution of variable composition, often physiologic, used to replace large volumes of ultrafiltrate during hemofiltration or hemodiafiltration. Replacement fluid may be given as predilution or postdilution.

Transmembrane pressure: The hydrostatic pressure gradient across the membrane. This is the driving force that causes ultrafiltration.


Bellomo, Rinaldo, and Claudio Ronco. "Nomenclature for continuous renal replacement therapies." Critical Care Nephrology. Springer Netherlands, 1998. 1169-1176.

Neri, Mauro, et al. "Nomenclature for renal replacement therapy in acute kidney injury: basic principles." Critical Care 20.1 (2016): 1-11.

Locatelli, Francesco, et al. "Dialysis dose and frequency." Nephrology Dialysis Transplantation 20.2 (2005): 285-296.

Claure-Del Granado, Rolando, et al. "Effluent volume in continuous renal replacement therapy overestimates the delivered dose of dialysis." Clinical Journal of the American Society of Nephrology 6.3 (2011): 467-475.

Clark, William R., et al. "Dose determinants in continuous renal replacement therapy." Artificial organs 27.9 (2003): 815-820.

O'Reilly, Philip, and Ashita Tolwani. "Renal Replacement Therapy III: IHD, CRRT, SLED." Critical care clinics 21.2 (2005): 367-378.