What is hyperosmolar hyperglycemic state? Hyperosmolar hyperglycemic state (HHS) is a diabetes emergency in which blood sugar climbs extremely high and the body becomes severely dehydrated. It happens most often in people with type 2 diabetes, often during another illness. Unlike diabetic ketoacidosis (DKA), it usually involves few or no ketones. HHS needs emergency care right away.
Key takeaways
- MedlinePlus says HHS involves extremely high blood sugar, extreme dehydration, and, in many cases, decreased alertness. Ketones are usually absent, and any ketone buildup is mild compared with DKA.
- The American Diabetes Association (ADA) lists criteria for HHS that include blood sugar of 600 mg/dL or higher, very concentrated blood, and no significant ketones or acid buildup.
- ADA says HHS usually develops over days to a week, and a change in thinking is common.
- A 2024 consensus report says infection is the main trigger in 30% to 60% of people with HHS, most often urinary tract infections and pneumonia. Older age, limited access to water, and running out of diabetes medicines also raise the risk.
- Call 911 for confusion, decreased alertness, a seizure, trouble speaking, or loss of muscle function in someone with diabetes. ADA says suspected HHS needs hospital care right away.
What Happens in the Body During HHS?
When blood sugar is high, MedlinePlus explains, the kidneys try to make up for it by letting the extra glucose leave the body in urine. That also makes the body lose water. If a person does not drink enough water, or drinks sugary fluids and keeps eating carbohydrates, they become very dehydrated. The kidneys can then no longer get rid of the extra glucose, so blood sugar can climb very high, sometimes to more than 10 times the normal level.
The water loss also makes the blood more concentrated than normal. This is called hyperosmolarity. It means the blood has a high concentration of salt (sodium), glucose, and other substances. MedlinePlus says this draws water out of the body's other organs, including the brain.
Ketones can build up in HHS too, but MedlinePlus says this is unusual and mild compared with DKA.
Doctors confirm HHS with blood tests. ADA lists these criteria in its 2026 Standards of Care, and all of them must be met. They are hospital tests, not numbers to check at home.
| What doctors measure | ADA 2026 criteria for HHS |
|---|---|
| Blood sugar | 600 mg/dL or higher |
| How concentrated the blood is (osmolality) | Calculated effective serum osmolality above 300 mOsm/kg, or total serum osmolality above 320 mOsm/kg |
| Ketones | No significant ketones: blood beta-hydroxybutyrate below 3.0 mmol/L or a urine ketone strip below 2+ |
| Acid in the blood | No acidosis: pH 7.3 or higher and bicarbonate 15 mmol/L or higher |
Our guide to what a dangerous blood sugar level is explains other readings that need attention.
How Is HHS Different From DKA?
DKA and HHS are both hyperglycemic crises, which ADA calls serious, acute, and life-threatening emergencies. They share many signs. A 2024 consensus report on hyperglycemic crises from the ADA and four other diabetes organizations says both may cause frequent urination, strong thirst, weight loss, vomiting, dehydration, and changes in thinking. ADA's 2026 Standards of Care say the two often show up at the same time, and about one-third of hyperglycemic emergencies have features of both.
| DKA | HHS | |
|---|---|---|
| How fast it develops | Over hours to days | Over days to a week |
| Alertness | Usually alert | A change in thinking is common |
| Blood sugar | 200 mg/dL or higher, or a known history of diabetes | 600 mg/dL or higher |
| Ketones and acid | High ketones and acidic blood | No significant ketones or acidosis |
| Other features | Nausea, vomiting, stomach pain, and deep, fast breathing | Often occurs alongside another sudden illness |
| Who it affects most | More common in young people with type 1 diabetes | Reported more often in older adults with type 2 diabetes |
The consensus report says most DKA events occur in people with type 1 diabetes (70.6%). HHS events are more common in people with type 2 diabetes (88.1%) and in adults ages 45 to 64 (47.5%).
HHS is also more deadly. With treatment, the consensus report says deaths from DKA have fallen below 1%, but deaths remain 5 to 10 times higher in people with HHS. ADA says the only study to look at deaths in mixed DKA-HHS found them higher than with either condition alone. Our main guide explains what diabetic ketoacidosis is.
What Are the Warning Signs of HHS?
MedlinePlus lists these symptoms:
- Increased thirst and urination, at the beginning
- Feeling weak
- Nausea
- Weight loss
- Dry mouth and dry tongue
- Fever
- Seizures
- Confusion
- Coma
MedlinePlus says symptoms may get worse over days or weeks. Other symptoms can include loss of feeling or function of muscles, problems with movement, and trouble speaking. Not everyone has every sign. Because a change in thinking is common, family members and care partners should know these signs too.
Trouble speaking and loss of muscle function can also be signs of a stroke, and MedlinePlus lists stroke as one possible trigger of HHS. Do not try to sort out the cause at home. Call 911. Our guide to whether diabetes causes strokes explains stroke warning signs.
For more on two early symptoms, see why diabetes causes frequent urination and whether diabetes causes dry mouth.
What Causes HHS?
ADA 2026 says HHS is typically triggered by a sudden illness. MedlinePlus says it may be brought on by:
- Infection
- Other illness, such as a heart attack or stroke
- Medicines that decrease the effect of insulin in the body
- Medicines or conditions that increase fluid loss
- Running out of, or not taking, prescribed diabetes medicines
Infection. The 2024 consensus report says infection is the main trigger in 30% to 60% of people with HHS, with urinary tract infections and pneumonia the most common. See whether recurrent UTIs can be a sign of diabetes.
Other illness. The consensus report also lists stroke, heart attack, surgery, and acute pancreatitis as common triggers.
Medicines. ADA 2026 says some medicine classes can affect how the body handles carbohydrates and lead to DKA or HHS. These include glucocorticoids (steroids), antipsychotic medicines, immune checkpoint inhibitors, and SGLT2 inhibitors. Never stop a prescribed medicine on your own. Ask your care team whether any of yours raise your risk. See what medications raise blood sugar and does prednisone raise blood sugar.
Who Is at Risk for HHS?
MedlinePlus says HHS is more often seen in people with type 2 diabetes whose diabetes is not under control. It can also happen in people who have not been diagnosed with diabetes. MedlinePlus lists these risk factors:
- A stressful event, such as an infection, heart attack, stroke, or recent surgery
- Heart failure
- Impaired thirst
- Limited access to water, especially in people with dementia or who are bedbound
- Older age
- Poor kidney function
- Diabetes that is not well managed, including not following the treatment plan
- Stopping or running out of insulin or other medicines that lower blood sugar
Older adults. The consensus report says fluid loss is a primary driver of HHS. It says HHS commonly occurs in older adults with blood sugar above target, who are at high risk of dehydration because of frequent urination, a weaker sense of thirst with age, and limited access to fluids. In frail or older adults, it says HHS alone and mixed DKA-HHS occur more often than DKA.
If you help care for an older adult with diabetes, ask the care team about a sick-day and hydration plan, especially if the person has heart failure, kidney disease, or memory problems. Our guide to how to manage diabetes in older adults covers more. For the wider risks of high blood sugar over time, see what happens if type 2 diabetes goes untreated.
How Is HHS Treated?
ADA 2026 says that because HHS involves greater fluid loss and is usually triggered by a sudden illness, people with suspected HHS should be evaluated and treated in the hospital right away. The consensus report says people with HHS, like those with severe DKA, should be treated in an intensive care unit (ICU).
MedlinePlus says treatment starts by correcting the water loss. This improves blood pressure, urine output, and circulation, and it lowers blood sugar too. Fluids and potassium are given carefully through a vein, and high blood sugar may be treated with insulin through a vein.
Doctors also look for the cause. MedlinePlus lists tests such as blood and urine cultures, a chest X-ray, an electrocardiogram (ECG), a urinalysis, COVID-19 and flu testing, and a CT scan of the head.
MedlinePlus notes that people who develop HHS are often already ill. If HHS is not treated right away, it says seizures, coma, or death may result. Untreated, HHS may also lead to shock, blood clots, brain swelling, and a buildup of acid in the blood called lactic acidosis.
After a hospital stay, ADA 2026 advises a follow-up visit with primary care, endocrinology, or a diabetes care and education specialist within 1 month, or in 1 to 2 weeks if diabetes medicines were changed. The consensus report says 10% to 20% of people are readmitted after DKA or another hyperglycemic crisis, mostly within 2 weeks. If you need a clinician, our guide to finding a diabetes doctor near you explains where to start.
How Can You Lower Your Risk of HHS?
MedlinePlus says controlling type 2 diabetes and recognizing the early signs of dehydration and infection can help prevent HHS. Steps to discuss with your care team:
- Have a sick-day plan for days when you are not eating well, as MedlinePlus suggests. Our diabetes sick day rules guide explains what to ask.
- Order your diabetes and other medicines in 3-month amounts when you can, so you are less likely to run out, as MedlinePlus suggests.
- Choose water or sugar-free fluids. MedlinePlus explains that drinking too little water, or drinking sugary fluids while blood sugar is high, can lead to severe dehydration. See whether drinking water lowers blood sugar.
- Check your blood sugar as your care team advises, especially when you are sick. See how to check your blood sugar at home.
- Call your care team early about signs of infection or about any medicine that may raise blood sugar or cause fluid loss.
- Ask about diabetes education. ADA 2026 recommends structured education on recognizing, preventing, and managing these emergencies for people with type 2 diabetes who have had one and for people at high risk. See diabetes self-management education and support.
Knowing your usual numbers makes very high readings easier to spot. Our A1C to blood sugar chart shows how average blood sugar relates to A1C.
When Should You Get Help?
Emergency: Call 911 or go to the emergency room for symptoms of HHS, as MedlinePlus advises. Act right away for confusion, decreased alertness, a seizure, trouble speaking, loss of muscle function, or signs of severe dehydration. The CDC's DKA guidance also advises emergency care if blood sugar stays at 300 mg/dL or higher.
Same day: Call your care team the same day if you are sick and your blood sugar stays above your target, if you are having trouble drinking enough fluids, or if you have run out of a diabetes medicine.
Prompt appointment: After any HHS hospital stay, book a follow-up visit within 1 month, or within 1 to 2 weeks if your medicines changed, as ADA 2026 advises. Also book a visit to make a sick-day plan, set up 3-month medicine refills, or review medicines that may raise blood sugar or cause fluid loss.
Frequently Asked Questions
What is the difference between DKA and HHS?
DKA involves high ketones and acidic blood, and ADA 2026 says it develops over hours to days. HHS involves blood sugar of 600 mg/dL or higher, very concentrated blood, and no significant ketones or acidosis, and it develops over days to a week. ADA says about one-third of hyperglycemic emergencies have features of both.
What blood sugar level is HHS?
ADA 2026 lists blood sugar of 600 mg/dL or higher as one of the criteria for HHS, along with very concentrated blood and no significant ketones or acidosis. MedlinePlus says blood sugar in HHS can reach more than 10 times the normal level. Doctors confirm HHS with blood tests.
Can people with type 1 diabetes get HHS?
Yes, though it is less common. The 2024 consensus report says both DKA and HHS can occur at any age and with any type of diabetes. It found that 88.1% of HHS events occurred in people with type 2 diabetes. MedlinePlus says HHS can also happen in people who have not been diagnosed with diabetes.
How long does HHS take to develop?
ADA 2026 says HHS usually develops over days to a week, compared with hours to days for DKA. MedlinePlus says symptoms may get worse over days or weeks. Call your care team early if you are sick and your blood sugar keeps rising.
Can HHS be fatal?
Yes. MedlinePlus says that if HHS is not treated right away, seizures, coma, or death may result. The 2024 consensus report says deaths in HHS remain 5 to 10 times higher than in DKA, where treatment has lowered deaths to below 1%. Call 911 for any warning signs.
How can HHS be prevented?
MedlinePlus says controlling type 2 diabetes and recognizing early signs of dehydration and infection can help prevent HHS. It suggests a sick-day plan for days when you are not eating well and ordering medicines in 3-month amounts so you are less likely to run out. Ask your care team how these fit your plan.
References
- MedlinePlus. “Diabetic Hyperglycemic Hyperosmolar Syndrome.” Accessed October 2026.
- Centers for Disease Control and Prevention. “Diabetic Ketoacidosis.” Accessed October 2026.
- American Diabetes Association Professional Practice Committee for Diabetes. “16. Diabetes Care in the Hospital: Standards of Care in Diabetes—2026.” Diabetes Care. 2026;49(Suppl. 1):S339–S355. Accessed October 2026.
- American Diabetes Association Professional Practice Committee for Diabetes. “6. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes—2026.” Diabetes Care. 2026;49(Suppl. 1):S132–S149. Accessed October 2026.
- Umpierrez GE, Davis GM, ElSayed NA, et al. “Hyperglycemic Crises in Adults With Diabetes: A Consensus Report.” Diabetes Care. 2024;47(8):1257–1275. Accessed October 2026.
- Pasquel FJ, Umpierrez GE. “Hyperosmolar Hyperglycemic State: A Historic Review of the Clinical Presentation, Diagnosis, and Treatment.” Diabetes Care. 2014;37(11):3124–3131. Historical context only. Accessed October 2026.
Next Steps
Ask your care team for a written sick-day plan that covers fluids, blood sugar checks, your medicines, and when to call 911. If you help care for an older adult with diabetes, share the plan with everyone who helps and keep water within easy reach.
For adults with type 2 diabetes or prediabetes who want lifestyle education alongside clinical care, the Done With Diabetes™ program offers a natural protocol for type 2 diabetes that organizes learning about meals, movement, sleep, and stress. It does not diagnose or treat HHS or diabetes, interpret glucose results, change medicines, or replace emergency or clinical care.