A diabetic patient says he doesn't feel well in the clinic. You check his blood sugar with a glucometer. What is the most likely reading?

An acute 'I don't feel well' presentation in a diabetic patient in the dental chair (especially with vague malaise, possible sweating/shakiness/confusion) is a classic presentation of hypoglycemia, which is the more immediately dangerous and rapidly progressive emergency compared to hyperglycemia (which usually develops more gradually and is less likely to cause sudden acute symptoms in a single chair-side moment). A glucometer reading of 40 mg/dL reflects significant hypoglycemia requiring immediate management (oral glucose if conscious).