**1. Introduction**

In hemodialysis treatment of end-stage renal disease, the patient's blood is separated from dialysis fluid by a semipermeable membrane in the dialyzer. In cases of microbiological contamination of dialysis fluid, it contains pyrogenic substances, such as endotoxins, which would cause fever in the patient if they entered the patient's blood. However, the permeability of the membrane for endotoxins is limited. Therefore, the dialyzer membrane is often called the last line of protection against endotoxins when the microbiological quality of dialysis fluids is discussed. The simplicity of this visualization makes it easy to believe that the underlying concept would have been understood. However, comprehensive and quantitative evaluations and specific definitions of this concept are missing or remain vague. Different situations during a dialysis treatment need to be evaluated in a differentiated manner to account for specific contamination levels, membrane conditions, and fluid flow circumstances. Especially, trends of

#### *Updates on Hemodialysis*

using online prepared fluid during online priming, online bolus, and online rinse-back need to be examined, since that fluid is filtered across the dialyzer membrane before it is infused into the patient. A verifiable quantitative requirement for the dialyzer membrane to serve as the last line of protection against endotoxins has not yet been defined. This work develops a proposal of how such a requirement could look like.
