A blood sugar of 300 mg/dL is significantly above normal range and requires prompt attention. For context, a normal fasting blood sugar is under 100 mg/dL, and even for people with diabetes, anything above 180 mg/dL after eating is considered too high. At 300, your body is under real stress, and the steps you take in the next few hours matter.
Why 300 mg/dL Is a Serious Reading
At 300 mg/dL, your blood contains roughly three times the amount of sugar it should. Your kidneys, which normally reabsorb glucose and send it back into your bloodstream, get overwhelmed and start dumping sugar into your urine instead. That pulls water along with it, which is why extreme thirst and frequent urination are two of the hallmark signs of very high blood sugar. This cycle of losing fluid can quickly lead to dehydration, which makes everything worse.
The CDC specifically lists a blood sugar that stays at 300 mg/dL or above as a reason to go to the emergency room or call 911. This is especially true if you’re also experiencing vomiting, difficulty breathing, or fruity-smelling breath, all of which signal a dangerous complication called diabetic ketoacidosis (DKA).
Symptoms You May Be Feeling
Many people don’t notice symptoms of high blood sugar until they cross 250 mg/dL. At 300, you’ll likely experience some combination of the following:
- Intense thirst that doesn’t go away no matter how much you drink
- Frequent urination, sometimes every 30 to 60 minutes
- Blurred vision
- Headache
- Fatigue or weakness
Some people at this level feel surprisingly fine, which can be misleading. The absence of symptoms doesn’t mean the reading is harmless. If you’ve been running high for days or weeks, your body may have adjusted to the elevated sugar, masking the urgency of the situation.
The Immediate Danger: Ketoacidosis
The biggest short-term risk at 300 mg/dL is DKA. This happens when your body can’t use glucose for energy (usually because there isn’t enough insulin), so it starts breaking down fat at a rapid pace instead. That process produces acids called ketones, which build up in the blood and make it dangerously acidic.
DKA is most common in people with Type 1 diabetes, but it can also happen in Type 2 diabetes, particularly during illness or infection. Warning signs include nausea and vomiting, stomach pain, fruity-smelling breath, rapid heartbeat, and deep labored breathing. Confusion or disorientation means the situation is advanced. DKA is a medical emergency that can progress to loss of consciousness if untreated.
If you have any of those symptoms alongside a reading of 300, get emergency care immediately. Don’t wait to see if the number comes down on its own.
What to Do Right Now
Your first step at 300 mg/dL is to check for ketones. You can do this with an over-the-counter urine ketone test kit available at most pharmacies. The Mayo Clinic and CDC both recommend testing ketones any time your blood sugar is 240 mg/dL or above. If ketones are high, call your doctor or go to the ER. If ketones are negative or trace, you have a bit more room to manage the situation, but you still need a plan.
If you take insulin, follow whatever correction dose instructions your care team has given you. Don’t stack extra doses on top of each other; taking too much insulin too quickly can cause a dangerous low-sugar crash. Recheck your blood sugar every few hours to make sure it’s trending downward.
Drink water steadily. Your body is losing fluid through all that extra urination, and dehydration will keep your blood sugar elevated. Avoid sugary drinks, juice, and regular soda.
One important caution: do not exercise when your blood sugar is this high, especially if ketones are present. While physical activity normally helps lower blood sugar, at 300 it can actually push it higher. Your body is already in a stress state, and exercise adds to that stress, potentially worsening dehydration and ketone production.
What This Means If You Don’t Have a Diabetes Diagnosis
If you’ve never been told you have diabetes and a reading comes back at 300, this is almost certainly your body telling you something important. A fasting blood sugar above 125 mg/dL on more than one occasion typically leads to a Type 2 diabetes diagnosis. People with previously undiagnosed Type 1 diabetes often present with readings above 250 mg/dL. A reading of 300 with no prior diagnosis warrants medical evaluation as soon as possible, both to address the immediate high and to determine what’s causing it.
Damage From Staying High
A single spike to 300 that comes back down within a few hours is unlikely to cause lasting harm on its own. The real danger is when readings like this become frequent or sustained over days and weeks. Chronically elevated blood sugar damages blood vessels and nerves throughout the body. The small vessels in your eyes, kidneys, and feet are especially vulnerable.
Over time, this leads to complications like vision loss (retinopathy), kidney disease, and nerve damage in the hands and feet (neuropathy) that causes tingling, numbness, or pain. The large blood vessels are affected too, raising the risk of heart attack and stroke. If your meter regularly shows readings near 300, it means your overall blood sugar management needs a significant adjustment, whether that involves medication changes, dietary shifts, or both.
How to Recheck and Monitor
After taking corrective steps, recheck your blood sugar every four to six hours. You’re looking for a steady downward trend rather than a sudden drop. If your reading stays at 300 or above despite correction, or if it continues to climb, that’s a sign something more is going on, such as an infection, a missed insulin dose, a medication issue, or a malfunctioning insulin pump, and you need medical help.
Keep a record of what you ate, any medications you took, and how you were feeling. This information helps your care team figure out what triggered the spike and how to prevent it from happening again. Patterns matter more than any single reading, but 300 is high enough that even a one-time occurrence deserves follow-up.