I am mystified by this as well. 10993-5 says that for the MTT test specifically (in the annex) it’s 24 hours incubation, and I can’t find anything in 10993-5 or the FDA guidance either. I do know that USP recommends at least 48 hours incubation, so it’s possible they’re referencing that? But it’s for the elution method, not the MTT.