Was I taught the wrong thing in school?

As a recent grad and new med/surg nurse, I realize that there is a difference in textbook and real world nursing.

However, one of the things that my instructors drilled into our heads was the "sick day protocol" for diabetics. "Take your insulin, monitor blood glucose more frequently."

The source of my confusion is I had a pt with blood glucose of 309 at hs check. On the sliding scale prescribed, the pt would have gotten 3 units of Novolog (at home the pt would have take a lot more). Because the pt was NPO, my preceptor instructed me to hold the insulin. This pt was in pain, very stressed and had an infection, was going for a procedure in the morning. Seems like a perfect combo to raise the blood glucose to me.

So here is the question, do you hold the insulin or give it when your pt is short term NPO? Any insight would be greatly appreciated.

Thank you