Cortisol level influenced by prednisone?

how to determine if patient is truly having adrenal crisis if they are on chronic prednisone of 5 mg? morning cortisol off prednisone is 36. since our lab uses immunoassay, does it rule out adrenal crisis? was thinking it could cross react making spurious elevation? or does it make the cortisol level normal?

i am super confused if someone can guide me in the right direction in a concise manner. thank you!