Although a wide range of physical principles capable of separating different solutes exist in biochemistry (such as affinity, or size as well as charge retaining columns and others), the removal of uraemic solutes has been almost exclusively performed up to the present with membrane-based systems. Sir Thomas Graham, in the second half of the 1800s, defined the method of separating various fluids by diffusion through a membrane with the term ‘dialysis’[1]. Galen in the second century of our era already claimed that the skin resembles a sieve and ‘sweating purifies the body, … by low-effort exercise, baths and the summer heat’ [De Symptomatum Causis Libri III, Claudii Galeni Opera Omnia (II)][2], and ancient Romans used the skin as a natural membrane to rid their bodies of poisonous urinal substances in the Therms and public baths. Well into the 20th century, artificial kidneys, based on membrane devices were adopted and the pioneer work by Abel, Rowntree and Turner [3], as well as that of Haas [4], was followed by the rotatory drum dialyser of Willem Kolff [5] and the vertical drum one of Nils Alwall [6]. Finally, the hollow fibre dialysers gained adepts and a widespread use of cuprophane membranes for a very long period of time (from the 1970s to the 1990s) has been followed by the introduction of high-flux membranes that have invaded most of the dialysis units worldwide to the present. It became quite clear from the very beginning that membranes differ in their clearance capacities of the different solutes, basically depending on thickness and pore size. However, increasing the pore size and reducing thickness is almost forcedly associated to a water permeability increase. The open dialysate circuit settings used during the era of low-permeability membranes had to be secured by the addition of ultrafiltration controllers, which closed the dialysis circuit [7], and are mandatory when using high-flux membranes (highly permeable to water) particularly if convective techniques are utilized. Defining water permeability of a dialyser was considered important from the beginning and is even more important with the high-flux dialysers. Water permeability of a dialyser was defined by its ultrafiltration coefficient, which is displayed in the notice of the given dialyser. The coefficient of ultrafiltration (KUF) was first defined by the amount of fluid (V) in mL crossing the dialyser membrane per time (T) in hours and pressure (P) in mmHg: KUF=VT×P The perception that renal physicians have of KUF has changed over time. Senior nephrologists considered KUF as a constant and took it into account in dialysis prescription in the low-permeability era [8]; it was common to hear comments on the different KUF or ‘slope’ of one dialyser in regard to another one in clinics and the consequences that this might have to the treatment and to the patient. Among senior physicians, only those particularly interested on the topic knew that KUF was not always a constant as its value may vary over a certain range of filtration rate. Young nephrologists, who have only lived the ultrafiltration controller era, have just ignored KUF. They simply did not need it. Nevertheless, the importance of KUF of the early times has remained in many aspects, including the approval of new devices by the regulatory agencies such as the US Food and Drugs Administration (FDA) [9] or its equivalent in Europe, the European Medicines Agency (EMA), a prerequisite to use them in clinics in all these countries. Indeed, the recent randomized, controlled trials on haemodiafiltration [10–12] and particularly that of Maduell et al. [12] providing evidence that high convective volume may improve survival has given a renewed protagonism to KUF, as it influences the convective capacities of the dialysis setting. KUF remains, though, the old ‘grand inconnu’. In the present editorial comment, we want to present a refurbished KUF to society, going in-depth into the factors influencing KUF and its calculation, and then coming back with as simple as possible methods to obtain it for easy clinical use.
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Oct 8, 2024
Özant Helvacı + 6
Open Access
