When would you suspect an allergy to the dialysis membrane in patients who complain of pruritis during dialysis?

The short answer is no. Pruritus is so common with renal disease, and allergic reactions to dialyzers are uncommon. If the pruritus can confidently be documented to be only during dialysis and not at any other time, then it may be worth trying a different dialyzer but it would take a lot for me to be convinced that it was truly an allergic reaction. It would be nice to get an opinion from allergy experts on what they think about pruritus as a sign of an allergic reaction without rash or other symptoms.

I have had multiple discussions with my nephrology colleagues over the years who almost uniformly agree that dialysis-related pruritus is not allergic in origin. It is important to establish 2 things in this context (as noted by Dr. @Dr. First Last).

1. Is the pruritus limited to dialysis or immediate post-dialysis, or is it persistent since, as noted above, ESRD patients commonly have some level of pruritus that has no allergic connection?
2. When the term allergy is used, this implies an IgE-mediated mast cell reaction vs a nonallergic mast cell-mediated pruritus. Several studies have been published looking at serum tryptase (enzyme almost exclusively released by activated mast cells) that are elevated in proportion to the level of pruritus du...