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Is a blood sugar of 300 an emergency?
A blood sugar reading over 300 mg/dL (16.7 mmol/L) is not automatically an emergency. It becomes one when the number stays there, or when it comes with specific warning signs.
The CDC tells people with diabetes to call 911 or go to the emergency room if blood sugar stays at 300 mg/dL or above, breath smells fruity, vomiting prevents keeping food or drink down, breathing becomes difficult, or several signs of diabetic ketoacidosis appear together.
If that describes you now, stop reading and get help. Caregivers of an older adult with type 2 should start at section four; readers with no diagnosis, at section seven.
ℹ️ Medical Disclaimer: General information only. This does not diagnose, does not tell you how much insulin or diabetes medication to take, and does not replace emergency assessment. Those decisions belong with a board-certified endocrinologist, your diabetes care team, or emergency clinicians.
When 300 means call 911 — and when it doesn’t
Three situations produce the same number and carry different urgency.
A high reading on its own
One reading of 310 with no other symptoms is not the CDC’s trigger. The wording is precise: the reading that warrants the emergency room is one that stays at 300 mg/dL or above.
A high reading with any of these signs
Call 911 or go to the emergency room if any of these are present:
- Blood sugar stays at 300 mg/dL (16.7 mmol/L) or above
- Breath smells fruity
- Vomiting that prevents keeping food or drink down
- Trouble breathing
- Several signs of diabetic ketoacidosis at once
A reading that won’t come down
No authority publishes a safe number of hours at 300. If it holds across repeat checks, that is the trigger; don’t wait for a symptom. Meters outside the US read in mmol/L, and our blood sugar converter turns 16.7 into 300.
Why a high reading turns dangerous
Those signs are not arbitrary. Each marks a stage in a process already underway.
🔬 How It Works: Insulin moves glucose from the blood into cells. Without enough, the liver breaks down fat for fuel instead, producing acids called ketones. Made too fast, they build to dangerous levels and turn the blood acidic.
What happens when insulin runs short
Illness that disrupts eating and drinking is the most common trigger, then missed insulin, a clogged pump, or the wrong dose. Some diuretics and corticosteroids set it off too.
Why the symptoms show up in that order
CDC lists the early signs as intense thirst and urinating far more than usual. Later signs arrive fast: deep laboured breathing, dry mouth, flushed face, fruity breath, exhaustion, nausea, vomiting, and stomach pain.
Diabetic ketoacidosis is most common in type 1, though type 2 develops it too. With no acute signs, the earlier symptoms of high blood sugar are worth knowing.
DKA and HHS are two different emergencies
Checking yourself against the wrong one is how people miss the second.
| Feature | Diabetic ketoacidosis | Hyperglycemic hyperosmolar syndrome | Key clinical detail |
|---|---|---|---|
| Affects | Type 1 mostly | Type 2, older adults | Type shifts which to expect |
| Ketones | Present, high | Absent or mild | Negative strip doesn’t mean safe |
| Glucose | From 200 mg/dL | 600 mg/dL (33.3 mmol/L)+ | DKA figure is lower than expected |
| Onset | Hours to a day | Days to weeks | HHS builds slowly, gets missed |
Features per CDC and MedlinePlus; thresholds per the 2024 international consensus report.
How HHS is different
Hyperglycemic hyperosmolar syndrome means very high glucose with severe dehydration and no significant ketones. Thirst and heavy urination come first, then weakness, fever, confusion, seizures, and loss of consciousness.
Who is most at risk
Older age, impaired thirst, limited access to water, poor kidney function, and infection all raise it — reasons blood sugar targets for adults over 65 differ.
⚠️ Clinical Warning: A negative ketone strip in an older adult with type 2 does not mean safe. HHS produces little ketosis, and confusion alongside a very high reading is an emergency whatever the strip shows.
Why a reading under 300 doesn’t always mean you’re safe
The threshold cuts both ways, and this is where most articles stop.
📊 Clinical Data Point: DKA is diagnosed at a glucose of 200 mg/dL (11.1 mmol/L) or above — or at any glucose level in someone with known diabetes. Source: Hyperglycemic Crises in Adults With Diabetes: A Consensus Report, Diabetes Care, 2024.
What the 2024 criteria actually say
That figure was lowered, and diabetes history added alongside it, because the old threshold missed real cases. Glucose alone never confirms DKA; the criteria also require ketosis and acidosis, measured clinically.
If you take an SGLT2 inhibitor
SGLT2 inhibitors flush glucose out through the urine, holding the number down while ketoacidosis develops underneath. The FDA’s ketoacidosis warning for this class followed reports where glucose was only slightly elevated.
What to check while you’re deciding
Testing for ketones
Check for ketones whenever you are sick or your blood sugar reaches 250 mg/dL (13.9 mmol/L) or above, rechecking every four to six hours. Many articles still cite 240, an older figure CDC’s diabetic ketoacidosis page has updated. If ketones appear, call your doctor and expect hospital treatment.
If you’re ill
CDC’s sick-day guidance is specific: keep taking insulin and diabetes pills as usual, test every four hours, drink water, eat normally, and check your temperature twice daily. If meals are impossible, aim for roughly 50 grams of carbohydrate every four hours.
⚠️ Clinical Warning: Nothing here tells you how much insulin to take. Correction doses stacked without supervision are a common route from a manageable high to a dangerous low. Dosing belongs with your prescribing clinician.
✅ Patient Action: Ask your diabetes care team: “What should my sick-day plan say, and at which ketone result do you want a phone call?”
If you don’t have a diabetes diagnosis
Why this can be the first sign
Sometimes DKA is the first noticeable sign of diabetes in someone never diagnosed, and HHS can appear in people never tested.
What to do without a diabetes team
Every CDC sign above applies with or without a diagnosis, and a first reading over 300 warrants same-day contact even if you feel well. High blood sugar without diabetes has several causes.
✅ Patient Action: Tell an urgent care or primary care physician: “I had a reading over 300 mg/dL and have never been tested for diabetes — what testing do I need, and how soon?”
Common questions about blood sugar over 300
1. Is a blood sugar of 300 an emergency?
Not automatically. It becomes one when the reading stays at 300 or above, or comes with fruity breath or vomiting.
2. What should I do if my blood sugar is over 300?
Check ketones, keep taking prescribed medicines, and go to the emergency room if any CDC warning sign appears. Contact your diabetes team.
3. When should I check for ketones?
Whenever you are sick or blood sugar reaches 250 mg/dL or above, rechecking blood sugar and urine ketones every four hours.
4. What blood sugar level means DKA?
DKA requires glucose of 200 mg/dL or above, or any level with known diabetes, plus ketosis and acidosis confirmed clinically.
5. Can you have DKA with normal blood sugar?
Yes. Euglycemic DKA occurs at near-normal glucose, most often with SGLT2 inhibitors. Ask your prescribing clinician about ketone testing.
6. What’s the difference between DKA and HHS?
DKA involves ketones and acid buildup, usually type 1. HHS means severe dehydration without significant ketones, usually older type 2 adults.
7. What blood sugar level causes HHS?
HHS is diagnosed at a plasma glucose of 600 mg/dL or above, with no significant ketones present.
8. How long can blood sugar stay at 300 before it’s dangerous?
No authority publishes a safe number of hours. CDC’s trigger is a reading that stays at 300 or above, plus warning signs.
9. What are the warning signs of DKA?
Early: intense thirst and heavy urination. Later: deep laboured breathing, fruity breath, dry mouth, nausea, and stomach pain.
10. Should I go to the ER for 300 with no symptoms?
Not on the number alone. CDC’s triggers are persistence at 300 or above plus companion signs. Call your diabetes team.
11. Can high blood sugar be the first sign of diabetes?
Yes. DKA is sometimes the first noticeable sign of undiagnosed diabetes. Seek same-day evaluation if you have never been tested.
What to do with this
The meter starts the decision; the warning signs finish it — and a number below 300 is not a clearance. Ask your diabetes care team to write your sick-day plan this week, while nothing is wrong. Our glucose levels chart covers every other result.
About this content
How this article was put together: researched from recognised health sources, drafted with the help of AI tools, and edited by hand, with sources linked throughout.
Sameer Patel is the founder and editor of My Medicine Advisor. He is not a doctor or medical professional — before starting this site he worked in banking,…
Medical disclaimer
The content on MyMedicineAdvisor is provided for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Health information on this website should not be used to diagnose, treat, cure, or prevent any condition without guidance from a qualified healthcare professional. Always seek the advice of your doctor, physician, or another licensed healthcare provider with any questions you may have regarding a medical condition, symptoms, medications, or treatment decisions.
