What's The Importance Of Cortisol Test Prior To Surgery? I

What's the importance of cortisol test prior to surgery? I took tapering dose of Prednisone started at 50 mg for 5 days & decrease increment of 10 mg every 5 days, my last dose was 2 was ago. I'll be having spinal fusion in 2 days. My doctor wants me to do the cortisol test , is it that important?
Thank you.