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When dialysis fluid is contaminated with endotoxins, the dialyzer membrane is often referred to as the last line of protection to prevent endotoxins from entering the patient’s blood. However, a quantifiable requirement for this endotoxin retention property of the membrane has not yet been defined. The ANSI/AAMI/ISO 23500 standard series provides the framework for the microbiological quality of dialysis water, concentrates, and dialysis fluid, and defines the limit value for the non-pyrogenic endotoxin dose. After defining the boundary conditions of the endotoxin loading of the membrane by dialysis fluid and the patient’s non-pyrogenic endotoxin dose, quantifiable requirements for the endotoxin retention properties of a membrane, expressed as a dimensionless logarithmic retention value (LRV), were developed in this work. Based on standard dialysis fluid quality, the LRV should minimally be two for a protein-coated membrane after contact with patient blood and minimally be one for a protein-free pristine membrane during online priming before contact with patient blood. This work also presents the critical factors for endotoxin retention tests and shows that the defined LRV values are reached by membranes in modern dialyzers.