A blood sugar reading above 250 mg/dL is entering dangerous territory, and anything above 300 mg/dL requires urgent attention. But the full picture depends on whether the spike is a one-time event, how long it lasts, and what symptoms accompany it. A single high reading after a big meal is very different from a sustained level of 400 mg/dL with vomiting and confusion.
Normal Ranges and Where Danger Starts
To understand what’s dangerously high, it helps to know what’s normal. For someone without diabetes, fasting blood sugar sits below 100 mg/dL, and it stays under 140 mg/dL even two hours after eating. For people managing diabetes, the American Diabetes Association recommends a fasting target of 80 to 130 mg/dL and a post-meal reading below 180 mg/dL (measured one to two hours after you start eating).
Once blood sugar climbs above 180 mg/dL, your kidneys start spilling glucose into your urine, which pulls water along with it. That’s why frequent urination and intense thirst are the earliest signs of hyperglycemia. At this level, you’re not in immediate danger, but your body is already working harder than it should be.
Above 240 mg/dL, the American Diabetes Association recommends checking your urine for ketones if you have diabetes. Ketones are acids your body produces when it burns fat instead of glucose for energy, and their buildup can turn a high blood sugar episode into a medical emergency. Above 300 mg/dL, you should be actively managing the situation and watching closely for worsening symptoms. Readings above 400 mg/dL are a medical emergency regardless of how you feel.
Two Emergencies Caused by Extreme Blood Sugar
Dangerously high blood sugar can trigger two distinct, life-threatening conditions, and they look different depending on your type of diabetes.
Diabetic Ketoacidosis (DKA)
DKA happens most often in type 1 diabetes, though it can occur in type 2 as well. It develops when the body has so little usable insulin that cells can’t absorb glucose and start breaking down fat at a rapid rate. This floods the blood with ketones, making it dangerously acidic. DKA can begin at blood sugar levels as low as 200 mg/dL, which is why the number on your meter alone doesn’t tell the whole story. The combination of high glucose, high ketones, and acidic blood is what makes it deadly.
Symptoms escalate quickly: nausea and vomiting, stomach pain, breath that smells fruity or sweet, deep labored breathing, rapid heartbeat, and eventually confusion or loss of consciousness. If you or someone around you has high blood sugar plus vomiting or difficulty breathing, that’s an emergency room situation. DKA can develop in hours, especially during illness, missed insulin doses, or when diabetes is first diagnosed and hasn’t been treated yet.
Hyperosmolar Hyperglycemic State (HHS)
HHS is more common in type 2 diabetes and tends to develop gradually over days or weeks, often in older adults. Blood sugar in HHS frequently exceeds 600 mg/dL and can climb past 1,000 mg/dL. At these levels, the blood becomes extremely concentrated, pulling water out of organs including the brain. Unlike DKA, significant ketone buildup is unusual, so the warning signs are subtler: escalating dehydration, weakness, leg cramps, vision changes, and progressive confusion. Because it creeps up slowly, HHS is often caught late, which makes it particularly dangerous. The mortality rate is higher than DKA.
What High Blood Sugar Does During Illness
Even people without diabetes can experience dangerously high blood sugar during severe illness, surgery, or major physical stress like a heart attack. This is called stress-induced hyperglycemia, and it’s defined as blood sugar above 140 mg/dL in a hospitalized person without a prior diabetes diagnosis. Hospitals typically aim to keep these patients below 180 mg/dL and begin treatment when levels exceed that threshold. In cardiac care, guidelines recommend keeping blood sugar below 200 mg/dL while carefully avoiding lows, since both extremes worsen outcomes.
If you’ve been told your blood sugar was high during a hospital stay or emergency, it doesn’t necessarily mean you have diabetes. But it does signal that your body’s glucose regulation struggled under stress, and follow-up testing after recovery is worth doing.
Symptoms That Signal Immediate Danger
A high number on a glucose meter matters, but your symptoms matter more. Mild hyperglycemia (180 to 250 mg/dL) often causes increased thirst, frequent urination, blurry vision, headaches, and fatigue. These are warning signs to act on, not emergencies.
The symptoms that demand immediate medical attention are:
- Nausea or vomiting that prevents you from keeping fluids down
- Fruity-smelling breath, which signals ketone buildup
- Deep, labored breathing or hyperventilation
- Severe dry mouth and inability to stay hydrated
- Confusion, disorientation, or unusual drowsiness
- Abdominal pain paired with any of the above
A blood sugar of 350 mg/dL in someone who feels relatively fine and can drink water is a very different situation from a blood sugar of 250 mg/dL in someone who is vomiting and confused. Both need management, but the second person needs an emergency room.
When to Test for Ketones at Home
If you have type 1 diabetes, checking for ketones is critical any time your blood sugar stays above 250 mg/dL for more than two hours and isn’t responding to correction, or any time you’re sick regardless of your blood sugar reading. Illness can trigger ketone production even at moderately elevated levels. Urine ketone strips are inexpensive and available at most pharmacies, and blood ketone meters give faster, more accurate results.
A positive ketone test combined with high blood sugar means you should contact your care team or head to an emergency room, especially if you’re also vomiting. The combination makes it very difficult to self-manage at home because you can’t keep fluids or medications down.
The Danger of Blood Sugar That Stays High
A single spike to 250 mg/dL after a holiday meal isn’t going to damage your organs. The real danger from moderate hyperglycemia comes from chronically elevated levels over months and years. This is what the A1C test measures: your average blood sugar over roughly three months. The ADA recommends keeping A1C below 7% for most adults with diabetes, which corresponds to an average blood sugar of about 154 mg/dL.
When blood sugar runs consistently high, the excess glucose damages blood vessels throughout your body. The smallest vessels are hit first, which is why the eyes, kidneys, and nerves in your feet are the most vulnerable targets. Diabetic retinopathy, kidney disease, and peripheral neuropathy (numbness, tingling, and pain in the hands and feet) all develop gradually from years of poorly controlled blood sugar. Larger vessels are affected too, raising the risk of heart attack and stroke significantly. None of these complications happen overnight, but once damage is established, it’s often only partially reversible. This makes sustained blood sugar control far more important than panicking over any single reading.
Quick Reference by Blood Sugar Level
- Under 180 mg/dL (post-meal): Within target for most people with diabetes
- 180 to 250 mg/dL: Above target. You may notice thirst, frequent urination, and fatigue. Correct with your usual plan and recheck
- 250 to 300 mg/dL: High enough to test for ketones if you have type 1 diabetes. Increase fluid intake, correct, and monitor closely
- 300 to 400 mg/dL: Requires active management. If levels don’t drop within a couple of hours or symptoms worsen, contact your care team
- Above 400 mg/dL: Medical emergency territory, especially with symptoms like vomiting, confusion, or difficulty breathing
- Above 600 mg/dL: Extremely dangerous. Associated with hyperosmolar syndrome, which can cause seizures, coma, and death without treatment