Risks & emergencies

Dangerously high blood sugar

Above 250 mg/dl (13.9 mmol/L), blood sugar stops being a chart question and becomes a management question. This page covers what to do today, the two emergencies that can develop — DKA and HHS — and the warning signs that mean "now", not "tomorrow".

First: is the number real?

Meters misread high for mundane reasons — sugar residue on fingers is the classic. Wash your hands with plain water, dry them, and re-test with a fresh strip. If the meter just says HI, it is above what it can measure (usually ~600 mg/dl): treat that as an emergency reading, not a glitch.

If the high reading is confirmed

1. Drink water. High glucose drags fluid out through the kidneys; dehydration then pushes glucose even higher. Water, steadily, is the single most useful home measure.
2. Skip the carbohydrates for the next meals until readings come down.
3. Type 1 (or on an SGLT2 inhibitor): check ketones now. Urine strips or a blood ketone meter. Moderate or large ketones + high glucose = call your care team immediately, do not exercise, do not wait it out.
4. On insulin with a correction plan: use it as prescribed. Without one: this reading is your reason to call the practice today — not to experiment with doses.
5. Not diagnosed with diabetes? A confirmed reading above 250 — or above 200 with thirst, frequent urination and fatigue — meets diagnostic territory. Same-day medical contact is the right pace.

Careful with exercise: a walk helps at moderately elevated levels, but with ketones present, exertion pushes glucose and ketones up, not down.

The two emergencies behind the warning box

Diabetic ketoacidosis (DKA)

Mostly type 1. Without enough insulin the body burns fat instead of glucose and floods itself with acidic ketones. It can develop within hours, sometimes at glucose levels barely above 250. The give-aways: nausea and vomiting, abdominal pain, deep sighing breathing, fruity breath. DKA is fatal untreated and very treatable in hospital — speed is everything.

Hyperosmolar hyperglycemic state (HHS)

Mostly type 2, often older people, often during an infection. Glucose climbs extremely high (frequently above 600) over days while dehydration deepens; ketones stay low, so there is no fruity warning. Increasing drowsiness and confusion are the signal. HHS is rarer than DKA and more dangerous — emergency care, always.

Why was it so high?

Worth asking once you are safe: missed or mistimed medication, an infection (the most common hidden driver — highs often precede the fever), steroid medication, a much bigger meal than usual, or — if the highs are new and unexplained — diabetes that has not been diagnosed yet. Log the reading with context (journal), because the pattern across days is what your doctor can actually work with.