A blood sugar reading over 300 mg/dL is a serious situation that needs immediate attention. The CDC lists a blood sugar that stays at or above 300 mg/dL as a reason to go to the emergency room or call 911, especially if other warning signs are present. Your first steps depend on whether you’re experiencing additional symptoms beyond the high number itself.
Check for Emergency Warning Signs First
Before anything else, take stock of how you feel. Certain symptoms alongside a 300+ reading signal a potentially life-threatening condition called diabetic ketoacidosis (DKA), where your body starts breaking down fat too quickly and floods the blood with acids. Look for these red flags:
- Fruity-smelling breath
- Vomiting or inability to keep food or liquids down
- Difficulty breathing or rapid breathing
- Confusion, extreme drowsiness, or trouble staying alert
If you have even one of these symptoms, call 911 or get to an emergency room immediately. DKA can progress quickly, and waiting it out is dangerous. People with type 2 diabetes face a related emergency called hyperosmolar hyperglycemic state (HHS), which typically involves blood sugar above 600 mg/dL paired with severe dehydration and neurological changes like confusion, vision problems, or delirium. HHS develops more slowly than DKA but carries a high fatality risk, so any mental status changes with extreme blood sugar readings warrant emergency care.
Test for Ketones
If you don’t have emergency symptoms, your next step is testing for ketones. The American Diabetes Association recommends ketone testing whenever blood sugar reaches 250 mg/dL or higher. You can use urine ketone strips (available at most pharmacies without a prescription) or a blood ketone meter if you have one. Blood meters are more precise: a reading of 1.5 mmol/L or higher is considered elevated and means your body is producing dangerous levels of ketones even if you feel relatively okay.
If ketones are moderate to high, contact your doctor or go to urgent care. If ketones are negative or trace amounts only, you can take steps at home to bring your blood sugar down, but continue monitoring closely.
Start Drinking Water Immediately
When blood sugar is this high, your kidneys are working overtime to flush excess glucose through urine, which pulls a large amount of water out of your body. That’s why extreme thirst and frequent urination are hallmark symptoms. Replacing that lost fluid is one of the most important things you can do right now.
Drink plain water steadily, not all at once. Sip consistently over the next several hours. Unsweetened herbal tea and plain sparkling water (no added sugar) also count toward your fluid intake. Avoid anything with sugar, including fruit juice, regular soda, and sports drinks, as these will push your blood sugar even higher. If you’re feeling nauseated but can tolerate small sips, keep trying. Vomiting and inability to keep fluids down is one of the signs that tips this into an emergency.
Take a Correction Dose If You Use Insulin
If you’re on insulin, a correction dose is typically the most effective way to bring a 300+ reading down. Use the correction factor your doctor has prescribed for you. If you use an insulin pump, your pump’s bolus calculator can suggest a dose, but be cautious about a common problem called insulin stacking, where you give repeated correction doses before the previous one has finished working. Research shows that at least two-thirds of all insulin boluses involve some degree of stacking, so it’s a very easy mistake to make.
To avoid stacking, wait the full duration of action for your insulin (typically 3 to 5 hours for rapid-acting insulin) before giving another correction, unless your pump’s calculator is accounting for the insulin still active in your system. If your blood sugar isn’t coming down after one full correction cycle, that’s a sign something else is going on and you should contact your healthcare provider.
If you take oral diabetes medications but not insulin, you won’t have a correction dose to take. Focus on hydration, avoid eating carbohydrates, and call your doctor’s office for guidance. A reading over 300 that doesn’t come down within a few hours on oral medications alone may need medical intervention.
Do Not Exercise
This is counterintuitive, since exercise normally helps lower blood sugar. But at levels above 250 mg/dL with ketones present, physical activity can actually make things worse. The American Diabetes Association’s position statement on exercise is clear: do not perform any exercise if blood sugar is 250 mg/dL or higher and moderate-to-large amounts of ketones are present. At 350 mg/dL or above, the same rule applies. Exercise under these conditions can cause blood sugar and ketone levels to rise further, even with mild activity.
Once your blood sugar is back below 250 and ketones are negative, gentle movement like walking can help. But until then, rest and let fluids and medication do the work.
Figure Out What Caused the Spike
A reading over 300 usually has a specific trigger. Identifying it helps you prevent it from happening again and gives your doctor useful information. Common causes include:
- Illness or infection: Even a mild cold or urinary tract infection can send blood sugar soaring. Your body releases stress hormones during illness that directly raise glucose levels.
- Missed or insufficient insulin: A forgotten dose, an expired insulin vial, or an insulin pump site that’s kinked or has been in too long can all cause rapid spikes.
- Steroid medications: Prednisone and similar drugs prescribed for inflammation are notorious for dramatically increasing blood sugar, sometimes within hours of the first dose.
- High-carbohydrate meals: A large meal with significant carbs, especially without adequate insulin coverage, can push readings well above 300.
- Stress: Physical or emotional stress triggers hormone responses that raise blood sugar independently of what you eat.
Write down what you ate, any medications you took or missed, whether you’re feeling sick, and the timeline of your blood sugar readings. This log becomes essential for your next conversation with your healthcare team.
Monitor Closely Over the Next Few Hours
After taking a correction dose or starting to hydrate, recheck your blood sugar every 30 to 60 minutes. You’re looking for a downward trend. A slow, steady decline is ideal. If your reading hasn’t budged after two hours, or if it continues climbing, that’s a reason to seek medical care rather than continuing to manage at home.
Keep rechecking ketones as well, especially if they were present on your first test. Ketone levels should drop as blood sugar comes down. If they’re rising or holding steady while blood sugar remains elevated, the situation is moving in the wrong direction.
What to Tell Your Doctor Afterward
Even if you successfully bring your blood sugar down at home, a reading over 300 is worth reporting to your healthcare provider. The American Diabetes Association considers recurring fasting blood sugar above 300 mg/dL that doesn’t respond to outpatient treatment a criterion for hospital admission, so your doctor needs to know this happened.
Bring your blood sugar log showing the spike and how long it took to resolve, the ketone test results, any potential triggers you identified, and a list of all your current medications. If this is happening more than occasionally, your treatment plan likely needs adjustment, whether that means changing your insulin doses, your correction factor, your basal rate, or your oral medication regimen. A single isolated spike with a clear cause (like a missed dose during illness) is different from a pattern, and your doctor needs the data to tell the difference.