High blood sugar with diabetes: symptoms, what to do and when it's an emergency

What counts as high blood sugar with diabetes?

High blood sugar, or hyperglycemia, means glucose above your target range, and it happens when the body has too little insulin or cannot use insulin properly.^[1] For many adults, ADA's goals are 80 to 130 mg/dL before meals and below 180 mg/dL one to two hours after a meal starts, adjusted for each person.^[2] Mayo Clinic notes that high blood sugar usually does not cause symptoms until it is above 180 to 200 mg/dL.^[3]

This guide covers how to recognize a high, what to check, what can cause it and which signs mean emergency care. It does not cover changing your medicine doses, which belongs in the plan you agree with your care team.

What are the symptoms of high blood sugar?

The early symptoms of high blood sugar are frequent urination, increased thirst, blurred vision and feeling weak or unusually tired.^[3] ADA also lists high levels of glucose in the urine.^[1]

These symptoms develop slowly over several days or weeks.^[3] Some people who have had type 2 diabetes for a long time notice no symptoms at all, even when their blood sugar is high.^[3] A reading tells you more than how you feel, so it is worth checking when something seems off.

What causes blood sugar to run high?

Blood sugar runs high when there is not enough insulin, or when insulin is not working as well as usual. ADA's guide to high blood glucose lists these common causes:^[1]

• Not enough insulin in type 1 diabetes, or insulin that is less effective than it should be in type 2 diabetes.
• Eating more, or being less active, than planned.
• Stress from an illness such as a cold or flu.
• Other stress, such as family conflicts.
• The dawn phenomenon, a surge of hormones the body produces daily around 4 to 5 a.m.

Mayo Clinic adds not using enough insulin or other diabetes medicine, injecting insulin incorrectly or using expired insulin, an infection, an injury or surgery, and some other medicines, such as steroids.^[3] Naming the likely cause is useful information for your care team, not a reason for blame.

What can I do when my blood sugar is high?

Follow the steps for highs in the plan you agreed with your care team, recheck, and look for the cause. If you do not have a written plan, that is a good thing to ask for at your next appointment.

1. Recheck to confirm the reading.
2. Follow the steps for high readings in your care plan, including any medicine instructions your care team gave you.
3. Check for ketones when your plan says to. ADA advises checking your urine for ketones when blood sugar is above 240 mg/dL.^[1] CDC advises checking when it is 250 mg/dL or above, or whenever you are sick.^[4]
4. If ketones are present, ADA advises against exercising, because exercise can push blood sugar even higher.^[1]
5. Recheck later and write down what happened, so you can bring the pattern to your care team.

Mayo Clinic suggests contacting your care team if your blood sugar stays above 240 mg/dL despite treatment, if you have a fever lasting more than 24 hours, or if you keep having trouble staying in your target range.^[3]

When is high blood sugar an emergency?

High blood sugar becomes an emergency when it leads to diabetic ketoacidosis (DKA) or the hyperosmolar hyperglycemic state (HHS), which ADA describes as serious and life-threatening.^[2]

CDC's page on diabetic ketoacidosis advises going to the emergency room or calling 911 right away if:^[4]

• Your blood sugar stays at 300 mg/dL or above.
• Your breath smells fruity.
• You are vomiting and cannot keep food or drinks down.
• You are having trouble breathing.
• You have several signs and symptoms of DKA.

CDC describes early DKA symptoms as being very thirsty and urinating much more than usual. If DKA is not treated, more severe symptoms can appear quickly, including fast, deep breathing, dry skin and mouth, a flushed face, fruity-smelling breath, headache, muscle aches, being very tired, nausea and vomiting, and stomach pain.^[4] CDC treats high ketones as a medical emergency in their own right.^[4]

DKA is most common in type 1 diabetes, but people with type 2 diabetes can also develop it.^[4] CDC lists illness and missed or wrong insulin doses as the two most common causes.^[4] HHS involves severe dehydration, is usually triggered by an illness, and needs immediate assessment in hospital.^[2] ADA also lists SGLT2 inhibitors, steroids and insulin rationing among the factors linked to these crises.^[2] If any of these apply to you, ask your care team which warning signs matter most for you.

It helps to tell someone close to you what these signs look like. You might say: "If I'm vomiting and my sugar stays over 300, please call 911." This is an illustrative prompt, not a quote from a real patient. Use your own words.

How can I prepare for sick days and stressful times?

Ask your care team for a written sick-day plan before you need one. ADA's sick day planning page covers what to prepare in advance. ADA notes that illness, injury and surgery raise the risk of both high and low blood sugar, and that care teams give individual guidance on what to do and whom to call, including outside office hours.^[2]

A sick-day plan usually answers these questions:^[2]

• How often to check blood sugar when you are unwell.
• When to check for ketones.
• Which medicines might be paused if you cannot eat or drink. ADA describes care teams considering this for metformin and SGLT2 inhibitors, and for GLP-1 medicines during illnesses with stomach symptoms.
• Whom to call, and when to go to the emergency room.

How is a high reading different from a high A1C?

A single high reading shows one moment, while your A1C reflects your average over about 3 months. For what an A1C result means and how goals are set, read what your A1C result means. If highs keep happening even though you take your medicines as planned, read questions to ask when blood sugar stays high on medication.

Where can I find support when blood sugar runs high?

The right support depends on whether you need urgent care, a clearer plan for highs, or help working out a pattern over time. Compare diabetes resources to see what each service offers and where it is available. Explore diabetes patient questions for illustrative ways to describe what is happening. The mama health app is one place to ask about your condition and get answers grounded in medical science and the experience of others living with diabetes. Learn more about its diabetes support.