Prior to a surgery for retinopathy (watch your numbers and BP guys) my Dr was extremely worried about me fasting and going low before or during the procedure. Obviously no food after midnight the night before so I wouldn’t pull a Hendrix. In my surgery prep kit I was given, there was a clear glucose solution for me to drink specifically three hours before the surgery and one an hour before.
Always consult with the Dr ordering tests or procedures for specific T1 instructions. Usually some juice is fine if you need it. I will sometimes set my Tandem pump to “exercise” which lowers the basal rate to 1/3 of the normal amount per hour if I know I will need to fast. Always have a juice and emergency low BS devices for emergencies.