**3. How can endotoxin retention properties be characterized and measured?**

To measure endotoxin retention properties of a membrane, the concentrations on both sides of the membrane need to be put into relation. The dimensionless ratio of concentrations on the feed side and the filtrate side of the membrane is a practicable relation. Since feed and filtrate concentration typically differ by a few orders of magnitude, it is convenient to transform the ratio as a decadic logarithm into a more practical dimension called logarithmic retention value (LRV).

$$LRV = 10\log\frac{C(Feed)}{C(Filter)}\tag{1}$$
