With regard to patients with kidney failure, sometimes the time of the prayer begins while undergoing dialysis which takes three to four hours, as the process of filtering the blood is with the use of a machine, does this invalidate ablution?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Undergoing dialysis does not invalidate ablution since the blood which comes out of the body through ways other than the private parts does not invalidate ablution, according to the most correct opinion of the scholars as clarified in Fatwa 87720.

Allaah Knows best.