What blood sugar level is dangerous for a non-diabetic?

The thresholds

The danger levels are the same regardless of diagnosis — glucose does not behave differently because nobody has written the word "diabetes" in your notes.

• Above 200 mg/dL (11.1 mmol/L) — needs investigating. With classic symptoms, a random reading at this level is diagnostic of diabetes on its own.
• Above 240 mg/dL (13.3 mmol/L) with symptoms — same-day medical advice, and check for ketones if you can.
• Above 300 mg/dL (16.7 mmol/L) — urgent.
• Above 400 mg/dL (22.2 mmol/L) — emergency care.

What differs for someone without a diagnosis is the interpretation: a high reading is more likely to be either a first sign of undiagnosed diabetes, or a specific temporary cause.

What causes high readings without diabetes

• Corticosteroids — prednisone, dexamethasone, or an injected steroid for a joint or the spine. The effect is large and can last days to weeks after a single injection. This is the most common surprise on the list.
• Acute illness or infection. Inflammatory signalling induces temporary insulin resistance. A urinary tract infection or pneumonia can push someone well into the 200s.
• Surgery, trauma or critical illness — "stress hyperglycemia", common in hospital, usually resolves.
• Other medications — thiazide diuretics, some atypical antipsychotics (olanzapine, clozapine), immunosuppressants such as tacrolimus, high-dose niacin.
• Pancreatic problems — pancreatitis, pancreatic surgery, or damage from hemochromatosis.
• Hormonal conditions — Cushing's syndrome, acromegaly, hyperthyroidism.

The part that gets missed

Stress hyperglycemia during a hospital stay is usually treated as a passing inconvenience and forgotten on discharge. It should not be. People who develop it are at markedly higher risk of developing diabetes later, and it deserves follow-up testing rather than a shrug.

If your glucose was high during an illness or after surgery, ask for a fasting glucose or A1c a few weeks after you have recovered.

Also worth knowing: reactive hypoglycemia

Some people without diabetes get symptomatic lows a few hours after a meal — shaky, sweaty, anxious. Below 70 mg/dL (3.9 mmol/L) is hypoglycemia and below 54 mg/dL (3.0 mmol/L) is a medical emergency regardless of how well you feel. If that is your pattern rather than highs, it needs a different investigation.