Prednisone interacts with blood levels of cortisol so lab values that measure cortisol levels are meaningless unless your Prednisone dose is very low. An endocrinologist said there was nothing she could do unless I could maintain a 3 mg dose of Prednisone for an extended period of time without having to increase my dose.

I have read that some endocrinologists won't even consider checking a cortisol level unless you can be off Prednisone for a couple of days. My endocrinologist said 48 hours so that was when a morning 8 a.m. cortisol level was done. First I had to be able to stay on 3 mg of Prednisone. Second I couldn't take any Prednisone for 48 hours.

Time of day is critical because cortisol levels are variable throughout the day. A "morning 8 a.m. cortisol level" was done because cortisol levels are highest in the morning.
https://www.webmd.com/a-to-z-guides/cortisol-test
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There are other factors that need to be considered in order to correctly interpret cortisol levels. Since cortisol is the stress hormone even your level of stress needs to be considered. Not every doctor is qualified to interpret cortisol levels but an endocrinologist is your best bet. Correctly interpreting cortisol levels is an art because of all the factors that need to be considered. It is not a simple thing to do. The lab is a simple blood draw but interpreting the results is difficult.

My rheumatologist did a screening 8 a.m. cortisol level before referring me to an endocrinologist. An endocrinologist rechecked and verified my level but wasn't at all surprised that it was so low because I was on Prednisone for more than 12 years. The endocrinologist took over for my rheumatologist and gave me instructions about my Prednisone dose with the hope of getting me off Prednisone eventually.

There is an adrenal stimulating test called the Synacthen test that is more complicated. My endocrinologist didn't think it was needed unless I was completely off Prednisone and still symptomatic. My endocrinologist felt that a Synacthen test might decide if I needed to go back on Prednisone but it wouldn't be helpful in terms of getting me off Prednisone.
https://pathologytestsexplained.org.au/ptests-pro.php?q=Synacthen+short+test
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There weren't any promises made that I would be able to get off prednisone. There was some back and forth between the endocrinologist and rheumatologist to make sure I didn't need Prednisone for the treatment of my autoimmune conditions. Even my ophthalmologist got involved because I had a flare of uveitis after my first attempt to taper off Prednisone. I had to go back up to 60 mg of Prednisone after my first attempt to discontinue Prednisone.

There was a learning curve involved that took about 2 years for my doctors to find a way to get me off Prednisone. Much of it involved trying 2 different biologics to figure out what worked best for me.

Ultimately, my rheumatologist said it would be impossible to "optimally" treat all my autoimmune conditions. This was after my endocrinologist determined my cortisol level was "adequate." That was when I was given a choice between only one biologic or stay on Prednisone. I chose Actemra because it worked best for me compared to the other options. I have no regrets about the decision I made.