High blood sugar is common in diabetes, and a few highs are early signs of diabetic ketoacidosis or a hyperosmolar state. Here is what counts as high, what pushes glucose up, how guidelines describe sick days, and when they say to get urgent help.
A reading of 14 mmol/L (252 mg/dL) after a birthday dinner and the same reading on a day you can’t keep fluids down are not the same situation. Both are high blood sugar, or hyperglycemia, meaning too much glucose in the blood 5. But vomiting that stops you keeping fluids down is a sign guidelines treat as a reason for urgent help 2, 4.
A few highs are early signs of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS), which the ADA calls “serious, acute, and life-threatening hyperglycemic emergencies” 1. This article covers what counts as high, symptoms, causes, ketones and sick days, and then, in the guidelines’ own words, when to get urgent help. It draws on guidance for adults. Low blood sugar has its own article, Low blood sugar explained.
No single number is high for everyone. Diabetes Canada notes that “Target ranges for blood glucose and A1C can vary and depend on a person’s medical conditions and other risk factors.” Its usual targets are 4.0 to 7.0 mmol/L (72 to 126 mg/dL) before meals and 5.0 to 10.0 mmol/L (90 to 180 mg/dL) two hours after eating 6.
For CGM data, the ADA Standards of Care call readings above 10.0 mmol/L (180 mg/dL) level 1 hyperglycemia and readings above 13.9 mmol/L (250 mg/dL) level 2. For most adults, the goals are less than 25% of the time above 10.0 mmol/L and less than 5% above 13.9 mmol/L 1. Understanding time in range explains these targets.
Other numbers appear in safety guidance: 11.1 mmol/L (200 mg/dL) in the ADA’s guidance on checking ketones 1, about 14 mmol/L (250 mg/dL) in Diabetes Canada’s and Joslin Diabetes Center’s 7, 8, and 16.7 mmol/L (300 mg/dL) in emergency lists 4, 9. mmol/L to mg/dL explains the two units.
High blood sugar is often silent at first. Mayo Clinic says it “usually doesn’t cause symptoms until blood sugar (glucose) levels are high”, above 10 to 11.1 mmol/L (180 to 200 mg/dL), and that symptoms develop over days or weeks 2. Cleveland Clinic says many people have no symptoms until their blood sugar is 13.9 mmol/L (250 mg/dL) or higher 5.
Early signs include passing more urine, thirst, blurred vision, and weakness or unusual tiredness 2, as well as hunger and headache 5. When glucose stays high for longer, Cleveland Clinic lists weight loss, yeast and skin infections, and cuts or sores that heal slowly 5.
Fruity-smelling breath, dry mouth, abdominal pain, nausea, vomiting, shortness of breath and confusion suggest ketones are building up 2. Those signs are covered under DKA and urgent help below.
Glucose climbs when there is too little insulin, or insulin works less well, for what the body is facing 2.
Mayo Clinic lists not using enough insulin or other diabetes medicine, injection technique problems and expired insulin 2. The CDC adds missed doses and a clogged insulin pump 4, and Cleveland Clinic adds insulin damaged by heat or cold 10. In a study of 164 adults admitted with DKA to a hospital in Atlanta, stopping insulin was the leading cause, in 68% of cases. Reasons included not being able to afford insulin, feeling sick and being away from their supply 11.
Illness and stress trigger hormones that can raise blood sugar 2. Infection is the most common trigger of DKA worldwide, behind 14% to 58% of cases, and the major trigger of HHS in 30% to 60% of people 3. Mayo Clinic names pneumonia and urinary tract infections as examples 9.
Mayo Clinic and Cleveland Clinic both list emotional stress 2, 5. Stress and blood sugar explains how stress hormones push glucose up.
Glucocorticoids, steroid medicines such as prednisone, make the liver release more glucose and cut glucose uptake in muscle and fat, reducing insulin sensitivity by as much as 60% in healthy volunteers 12. With a single morning dose, the authors of a Cleveland Clinic Journal of Medicine review report that glucose tends to rise from mid-morning until bedtime, while fasting glucose is often unaffected 12. The ADA also names antipsychotics, immune checkpoint inhibitors (a type of cancer treatment) and SGLT2 inhibitors among medicines that can contribute to DKA and HHS 1.
Cleveland Clinic lists carbohydrate not matched by insulin, insulin poorly timed for a meal, less activity than usual and the early-morning rise known as the dawn phenomenon 5. Same meal, different spike and The dawn phenomenon cover these in detail.
Without enough insulin, cells can’t use glucose for energy, so the body breaks down fat and makes ketones, which are acids 2, 4. When ketones build up and the blood turns acidic, that is DKA. In HHS, glucose climbs very high and the body becomes severely dehydrated, usually with few ketones 13.
DKA is most common in type 1 diabetes, but people with type 2 diabetes can develop it too 4. Between 6% and 21% of adults with type 1 diabetes are diagnosed when they arrive with DKA 3. A 2024 international consensus report defines DKA by three findings together: glucose of 11.1 mmol/L (200 mg/dL) or more, or known diabetes; a blood ketone (beta-hydroxybutyrate) level of 3.0 mmol/L or more, or urine ketones of 2+ or more; and acid blood 3, 14.
The time from first symptoms to hospital may be hours to a few days 3. About 10% of people with DKA have glucose below 11.1 mmol/L (200 mg/dL), called euglycemic DKA, which has become more common with SGLT2 inhibitors, a class of diabetes pills 3, 14. Diabetes Canada warns: “A normal or mildly elevated blood glucose level does not rule out diabetic ketoacidosis in certain conditions, such as pregnancy or with SGLT2 inhibitor use.” 7
HHS mainly affects people with type 2 diabetes, typically adults over 65 13. It builds over days or weeks 3, often with glucose above 33.3 mmol/L (600 mg/dL) 2. With treatment, deaths from DKA have fallen below 1%, but deaths from HHS remain five to ten times higher 3.
Hyperglycemic crises are also becoming more common 1. In the United States, the rate of hospital admissions for DKA rose by an average of 6.3% a year from 2009 to 2014 and was highest in people under 45 15.
| DKA | HHS | |
|---|---|---|
| Who it mostly affects | Most common in type 1 diabetes; can occur in type 2 4 | Mainly type 2 diabetes, typically adults over 65 13 |
| How fast it develops | Hours to a few days 3 | Days or weeks 3 |
| Glucose | 11.1 mmol/L (200 mg/dL) or more, or known diabetes; sometimes lower 3 | 33.3 mmol/L (600 mg/dL) or more 14 |
| Blood ketones | 3.0 mmol/L or more 14 | Below 3.0 mmol/L 14 |
| Typical signs | Nausea, vomiting and abdominal pain in more than half; deep, rapid breathing 3 | Confusion, vision changes, weakness, extreme thirst 13 |
Illness raises the stakes. Diabetes Canada says DKA and HHS “should be suspected in people who have diabetes and are ill” 7. Its key messages for people with diabetes note that “When you are sick, your blood glucose levels may fluctuate and be unpredictable”, suggest checking glucose more often than usual, “for example, every 2 to 4 hours”, and say “Drink plenty of sugar-free fluids or water.” 7
The core of the guidance is a plan written with your care team before you need it. Diabetes Canada says it should cover which diabetes medicines to continue and which to stop for a while, how to adjust insulin if you use it, and “Advice on when to contact your health-care provider or go to the emergency room.” 7 The ADA’s sick-day guide adds when to check ketones, which over-the-counter medicines are safe, and what to eat and drink with vomiting or diarrhea 16.
On insulin, the ADA Standards say people on intensive insulin therapy “should not stop or hold their basal insulin even if not eating” 1, and Joslin’s sick-day guidelines say: “Always take your diabetes medicine unless your healthcare provider tells you not to.” 8 In the Atlanta study, feeling sick was one reason people gave for stopping insulin before DKA 11. Because some medicines are paused during illness and others are not 7, the details belong in your own plan.
The ADA says people at risk of DKA should measure ketones “in the presence of symptoms and potential precipitating factors (e.g., illness, missed insulin doses, eating disorders), particularly if glucose levels exceed 200 mg/dL (11.1 mmol/L).” 1 Diabetes Canada says people with type 1 diabetes whose glucose stays over 14 mmol/L before meals, or who have DKA symptoms, should check ketones, and that “Blood ketone testing is preferred over urine testing.” 7
In a six-month trial in 123 children, teens and young adults with type 1 diabetes, those using a blood ketone meter on sick days had 38 hospital admissions or emergency assessments per 100 patient-years, against 75 with urine testing 17. The ADA describes this as a reduction of about 50% 1.
Guidelines use different numbers but agree on the warning signs. Here is what they say, word for word. Your own sick-day plan may set different numbers for you.
The CDC says: “Go to the emergency room or call 911 right away if you’re having any of these signs:” 4
Mayo Clinic says to seek emergency care if your blood sugar is “higher than 300 milligrams per deciliter (mg/dL), or 16.7 millimoles per liter (mmol/L)” for more than one test, if “You have ketones in your urine and can’t reach your healthcare professional for advice”, or if “You have many symptoms of diabetic ketoacidosis.” 9
For HHS, Cleveland Clinic says: “If you have diabetes, you should call 911 or seek emergency medical help if you:” “Ever have a blood sugar level over 600 mg/dL” (33.3 mmol/L), “Are confused or disoriented”, “Have changes in your vision” or “Feel weak or paralyzed anywhere in your body.” 13
The ADA’s sick-day guide says: “After following your Sick Day Plan, call your doctor or go to the ER if you experience:” 16
If you take an SGLT2 inhibitor, symptoms count even when glucose looks normal. Diabetes Canada recommends that “Individuals treated with SGLT2 inhibitors with symptoms of DKA should be assessed for this condition even if BG is not elevated”, where BG means blood glucose 7.
The ADA calls CGM readings above 10.0 mmol/L (180 mg/dL) level 1 hyperglycemia and above 13.9 mmol/L (250 mg/dL) level 2 1. Diabetes Canada’s usual targets are 4.0 to 7.0 mmol/L before meals and 5.0 to 10.0 mmol/L two hours after eating, though yours may differ 6.
Guidelines point to a sick-day plan made in advance with your care team, covering medicines, insulin, fluids, ketone checks and when to get help 7, 16. The CDC says to go to the emergency room or call 911 right away for signs such as fruity breath, trouble breathing, or vomiting with no food or drinks staying down 4.
The CDC lists being very thirsty and urinating much more than usual as early signs. Later signs include fast, deep breathing, fruity-smelling breath, nausea, vomiting, stomach pain and being very tired 4.
Yes. About 10% of people with DKA have glucose below 11.1 mmol/L (200 mg/dL), called euglycemic DKA 3. Diabetes Canada notes that a normal or mildly high glucose does not rule out DKA in pregnancy or with SGLT2 inhibitor use 7.
Diabetes Canada says people with type 2 diabetes generally don’t need to postpone exercise because of high glucose, provided they feel well 18. For type 1 diabetes, it suggests testing ketones if glucose is above 16.7 mmol/L (300 mg/dL) and you feel unwell, and postponing vigorous exercise when blood ketones are 1.5 mmol/L or more, or urine ketones 2+ or more, until they are no longer elevated 18. Why a run can drop your glucose and a sprint can raise it explains how exercise type changes the response.
After a run of highs, Glyve’s day view shows how long your glucose line stayed above your own target range, with logged meals, insulin, exercise, sleep and stress on the same line. Logging by typing, voice or photo is quick on a day you feel unwell, and each estimate is a draft you check before saving. The week view shows whether highs cluster after certain meals or on stressful days. Glyve reads glucose from Apple Health, and Dexcom and FreeStyle Libre readings through those companies’ sharing services, which can arrive later than in your CGM app, so rely on your meter, CGM app and sick-day plan in the moment. Glyve shows what happened together; it doesn’t tell you what caused it. It is not a medical device and gives no advice.
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Glyve keeps glucose, food, insulin, exercise, sleep and stress on one timeline.
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