Blood Sugar of 300 mg/dL: When to Get Emergency Help
If your blood sugar stays at 300 mg/dL (16.7 mmol/L) or above, the CDC says to go to the ER or call 911. Check for ketones, and do not wait if you are vomiting, short of breath or very unwell.
What to do while you arrange care
- Test for ketones if you have strips or a meter. Large urine ketones, or blood ketones of 1.6 mmol/L or higher (ADA), mean seek emergency care now. The ketone levels chart has the bands.
- Call your care team or the number on your sick-day plan. They can tell you whether to go in.
- Do not drive yourself if you are dizzy, confused or vomiting. Ask someone, or call 911.
- Bring your supplies and list of medicines so the team can see what you use.
This page does not give medicine instructions. Please do not take extra medicine on your own because of a number. The right action depends on your type of diabetes, your treatment and your health.
Why 300 matters
Very high glucose pulls water out of the body, causing thirst and frequent urination and then dehydration. When insulin is short or illness adds stress, ketones can accumulate and tip into DKA. The CDC says two of the most common causes are illness and missed or incorrect insulin: missing shots, a clogged pump, or the wrong dose. Other causes include heart attack, stroke, injury, alcohol or drug use, and some medicines. Elevated ketones are a sign of DKA, a medical emergency.
After you are safe
Ask what led to it, what your sick-day plan should say, and which supplies to keep (ketone strips, a meter, glucose tablets, glucagon if prescribed). Keep a record in the blood sugar log. Read about DKA symptoms, ketones in urine and blood sugar of 250.
Frequently asked questions
Is a blood sugar of 300 an emergency?
The CDC says to go to the ER or call 911 if your blood sugar stays at 300 mg/dL (16.7 mmol/L) or above. A single reading of 300 is a signal to act: check ketones if you can, recheck, and call your care team. If you feel very unwell, do not wait.
What is 300 mg/dL in mmol/L?
16.7 mmol/L (300 ÷ 18).
Do I need to check ketones at 300?
The CDC advises testing for ketones when you are sick or your blood sugar is 250 mg/dL or higher, so yes. Elevated ketones are a sign of DKA, which the CDC calls a medical emergency that needs to be treated immediately.
Can I have 300 without symptoms?
Yes, sometimes, especially if you are used to high readings. That does not make it safe. Follow the CDC criteria and your care team’s plan.
What if I do not have diabetes and see 300?
A reading this high with no diagnosis needs medical attention promptly. Call your clinician or urgent care, and go to the ER if you feel unwell.
Information only, not medical advice. Your care team may set different targets for you, and only a lab test ordered by a clinician can diagnose diabetes or prediabetes. See our medical disclaimer.
Sources
- CDC: Diabetic Ketoacidosis
- ADA: DKA (Ketoacidosis) & Ketones
- ADA Standards of Care in Diabetes—2026, Section 6: Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises
Last reviewed: by the Diabetes Blogs editorial team. Information only, not medical advice.
