Which is the best test for diagnosis of adrenal insufficiency?

The diagnosis of Primary Adrenal Insufficiency is established by Cosyntropin test Coisol values of <18ug/dl sampled 30-60 minutes after ACTH stimulation will diagnose the disease. This is followed by plasma ACTH values to identify primary versus secondary Addison disease. Steroid auto-antibodies are marker for Autoimmune Adrenalitis. Fludrocoisone challenge test is done for Conn syndrome.