What 60 minute cortisol threshold value do you use for diagnosis of adrenal insufficiency after ACTH stimulation?

It depends on the assay. Different assays measure different things. Mass spectrometry is the most accurate available assay at present, and it measures ONLY cortisol. Immunoassays also detect other related compounds, and vary from assay to assay as to which ones. Discussed in PMID 25877990.

Based on our experience, we use a cortisol level >18 ug/dL at 60 minutes during the standard dose CST as the pass criteria. Husni et al., PMID 36060940

When Cortrosyn is given IV, 18 or greater. However, a published study showed that 16 was sufficient if injected IM.

Cutoff 18 is universally accepted as a normal response, but anything closer is also fine.

Depends on the assay. Older assays, the answer was 18 for someone undergoing a standard 250mcg IV cosyntropin stim test. These days, the assays have less cross-creativity thus the cutoff is lower - 14-15 is about right.