A blood sugar level above 250 mg/dL is entering dangerous territory, and anything above 300 mg/dL poses serious, immediate health risks. At 600 mg/dL or higher, blood sugar is life-threatening. The exact danger point depends on whether you have type 1 diabetes, type 2 diabetes, or no diabetes diagnosis at all, and on how quickly the spike occurred.
The Key Numbers to Know
Normal fasting blood sugar falls below 100 mg/dL. For people with diabetes, readings can climb well beyond that, and the risk increases on a sliding scale rather than at a single cutoff. Here’s how the danger zones break down:
- Above 250 mg/dL: High enough to start checking for ketones if you have type 1 diabetes. Symptoms like excessive thirst, frequent urination, and blurred vision are common.
- Above 300 mg/dL: Consistently dangerous. Your body struggles to function normally and organ stress begins to mount. Contact your care team if readings stay in this range.
- Above 600 mg/dL: This defines a condition called hyperosmolar hyperglycemic state (HHS), which is a medical emergency. Your blood becomes dangerously concentrated, and without treatment, it can lead to seizures, coma, or death.
These numbers apply to glucose meter readings. If your meter simply reads “HIGH” without a number, that typically means your blood sugar has exceeded the meter’s upper limit (usually around 500 to 600 mg/dL), and you should treat it as an emergency.
Two Types of Hyperglycemic Emergencies
Dangerously high blood sugar doesn’t look the same in every person. There are two distinct emergencies, and they tend to affect different populations.
Diabetic Ketoacidosis (DKA)
DKA happens when the body has so little usable insulin that cells can’t absorb glucose for energy. Instead, the body breaks down fat rapidly, producing acids called ketones. These ketones build up in the blood and make it dangerously acidic. DKA is most common in type 1 diabetes but can occur in type 2 as well.
What makes DKA tricky is that blood sugar doesn’t have to be astronomically high. It can develop at glucose levels as low as 200 mg/dL if ketone production and acid buildup are severe enough. The warning signs come on relatively fast, sometimes within hours:
- Fruity-smelling breath (a hallmark sign of ketone buildup)
- Nausea and vomiting
- Shortness of breath or rapid breathing
- Very dry mouth
- Confusion or difficulty staying alert
If you have a ketone meter, the NHS provides a helpful scale: a blood ketone reading under 0.6 mmol/L is normal, 0.6 to 1.5 mmol/L is slightly high and worth rechecking in two hours, 1.6 to 3.0 mmol/L means you’re at risk and should contact your diabetes care team, and above 3.0 mmol/L means DKA is likely and you need emergency care immediately.
Hyperosmolar Hyperglycemic State (HHS)
HHS is the other emergency, and it tends to develop more slowly, over days or even weeks. It’s most common in older adults with type 2 diabetes. Blood sugar climbs above 600 mg/dL, and the body tries to flush the excess glucose through the kidneys, causing extreme dehydration. The blood becomes so concentrated that the brain and other organs can’t function properly.
Unlike DKA, HHS doesn’t produce significant ketones. That means you won’t get the fruity breath or rapid acid buildup. Instead, the dominant signs are severe dehydration, confusion, weakness, and eventually loss of consciousness. Because the symptoms develop gradually, HHS often goes unrecognized until the person is critically ill. It carries a higher mortality rate than DKA, largely because of this delayed recognition.
What Happens Inside Your Body
Extremely high glucose doesn’t just sit in your bloodstream doing nothing. It actively damages tissue through several pathways at once. When blood sugar stays elevated, normal glucose metabolism becomes saturated. The body shunts excess glucose into alternative processing pathways that generate toxic byproducts, causing biochemical disruption in vulnerable organs.
The kidneys take an especially hard hit. High glucose triggers inflammation and structural changes in the kidney’s filtering units. Cells in the kidneys undergo functional and physical alterations that can become irreversible if the exposure lasts long enough. The brain is vulnerable too. In HHS, extreme dehydration causes brain cells to shrink, which can lead to seizures and coma. In DKA, the acidic blood environment impairs brain function directly.
The heart faces increased stress as dehydration thickens the blood and forces the cardiovascular system to work harder. Blood vessels stiffen, and the risk of clotting rises. This is why dangerously high blood sugar can trigger heart attacks and strokes, even in people who have never had cardiovascular problems before.
High Blood Sugar Without Diabetes
You don’t need a diabetes diagnosis to experience dangerously high blood sugar. During serious illness, major surgery, or severe infections like sepsis, the body floods the bloodstream with stress hormones that drive glucose levels up. This is called stress-induced hyperglycemia, and it’s more dangerous than it might sound.
Most research defines stress hyperglycemia as an admission blood sugar above 200 mg/dL (11.1 mmol/L) in someone without known diabetes. A large meta-analysis of sepsis patients found that this kind of spike is significantly associated with higher short-term mortality. The body simply isn’t adapted to those glucose levels, and the combination of high sugar plus acute illness creates compounding organ stress.
If you’re hospitalized and your blood sugar spikes despite never having diabetes, the medical team will manage it aggressively. For people outside the hospital, an unexpectedly high glucose reading during illness (especially above 300 mg/dL) warrants urgent medical attention even if you’ve never been told you have diabetes.
What to Do at Home When Readings Spike
If you check your blood sugar and see a number above 250 mg/dL, the first step is to test for ketones if you have a way to do so, especially if you have type 1 diabetes. Drink water steadily to help your kidneys flush excess glucose. Take your prescribed insulin if your care plan allows for correction doses.
The CDC outlines clear situations that call for emergency care:
- Ketones are present in your urine or blood
- You’re having trouble breathing
- You can’t keep liquids down for more than 4 hours
- You’ve had vomiting or severe diarrhea for more than 6 hours
- Your temperature has been above 101°F for 24 hours
Don’t wait to see if things improve on their own once these signs appear. DKA can progress from uncomfortable to life-threatening in a matter of hours.
What Emergency Treatment Looks Like
In the emergency room, the priority is rehydration through IV fluids, followed by carefully controlled insulin to bring glucose down gradually. The emphasis on “gradually” is important. Dropping blood sugar too fast creates its own dangers, particularly severe low blood sugar, which carries its own mortality risk. One major study found that ICU patients who experienced even moderate low blood sugar episodes from overly aggressive treatment had significantly higher death rates than those whose glucose was lowered more steadily.
During treatment, blood sugar is checked every hour. Once readings stabilize within the target range for three consecutive checks, monitoring is spaced to every two hours. Full stabilization over 12 to 24 hours allows spacing to every four hours. The total time in the hospital depends on the severity: mild DKA might resolve in 12 to 24 hours, while HHS can require several days of intensive care.
Recovery doesn’t end at discharge. After a hyperglycemic emergency, your diabetes management plan will likely need adjustment. The episode itself may have caused some degree of kidney stress or other organ strain that your care team will want to monitor in the weeks that follow.