There is no clear answer to this question. I would use RIFLE criteria to decide RRT. There are some suggestions to start RRT when the patient is classified as Injury on RIFLE. I've heard some also suggest beginning it in Risk on RIFLE.

Your question still remains THE BIG QUESTION. When to start CRRT? I would say after all that has been published (although no RCT has really compared Early vs Late with the same baseline AKI degree) that i would definitely NOT initiate CRRT in the absence of AKI and that the exact moment could probably be any AKIN score over 0 with oliguria (

Thanks.