If you have diabetes, it's very important to keep your blood sugar, or glucose, levels within a healthy range.
Here's why: If your glucose levels dip too low, you can experience symptoms of hypoglycemia, which include shakiness, a rapid heartbeat and lightheadedness.
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And if your blood sugar levels swing too high (called hyperglycemia), you could experience not just long-term health complications, but short-term problems like diabetic ketoacidosis (DKA), a serious condition that could possibly be fatal.
Find out what happens when your blood sugar levels are too high, as well as potential outcomes when they rise above 400 milligrams per deciliter (mg/dL).
If your blood sugar is over 400 (or persistently higher than 240), seek medical attention immediately, according to the Mayo Clinic.
What Are the Symptoms of High Blood Sugar?
In general, the American Diabetes Association (ADA) recommends shooting for a blood sugar level of 80 to 130 mg/dL before a meal and a level of less than 180 mg/dL after a meal. Still, because people's target blood glucose levels can vary, it's best to work with your doctor or health care provider to determine the right numbers for you.
If your blood sugar levels are too high, you can experience symptoms like frequent urination and increased thirst. Left untreated, hyperglycemia can develop into diabetic ketoacidosis, which occurs when your body is no longer able to use insulin to break down glucose for energy.
Without insulin, your body begins to break down fats, triggering the production of chemicals called ketones, according to the ADA. Your body can't harbor too many ketones, so it tries to flush them out of the body through the urine — but because the body can't always eliminate them all, they can continue to build up, causing diabetic ketoacidosis.
Causes of High Blood Sugar
High blood sugar can come about for a number of reasons. In general, though, if you have diabetes, you can experience high blood sugar episodes because the insulin in your body isn't able to adequately regulate the amount of glucose in your blood, per the Mayo Clinic.
According to the Cleveland Clinic, there are other reasons your blood sugar can spike when you have diabetes, including:
- You're not taking the right dose of insulin or diabetes medication.
- The amount of carbohydrates you're eating isn't balanced with your insulin medication dosage.
- You're less physically active.
- You're physically or emotionally stressed.
- You're taking steroid medications.
- You're experiencing the dawn phenomenon, which is when your body produces a surge of hormones early in the morning.
High blood sugar can also occur if you don't have diabetes, per the Cleveland Clinic. Causes of these spikes can include:
- Endocrine conditions like Cushing syndrome
- Pancreatic diseases like pancreatitis and cystic fibrosis
- Certain medications like diuretics and steroids
- Gestational diabetes
- Side effects from surgery
How Do I Know if I'm at Risk?
Testing your urine with a urine test strip will reveal the presence of ketones in your urine — high levels of these chemicals are a potential signal of diabetic ketoacidosis. Your doctor will tell you when you should perform the test, but the ADA says that, in general, you may want to check your urine for ketones when your blood sugar levels hit more than 240 mg/dL.
Other symptoms of diabetic ketoacidosis include nausea, difficulty breathing, an altered mental state and a fruity odor on the breath.
"In our clinic, if a person has blood sugar levels of over 300 mg/dL, we will check for urine ketones," says Susan Spratt, MD, an endocrinologist and associate professor of medicine at Duke University School of Medicine. "If their blood sugar levels are over 300 mg/dL — and certainly over 400 mg/dL — we're concerned that they may be going into diabetic ketoacidosis."
If your urine test reveals that ketones are present, call your doctor, who can give you further instructions. You may need to go to the emergency room to seek treatment right away. The ADA says that if you have ketones present in the body, you shouldn't exercise. In this case, exercise can cause your blood sugar levels to spike even higher.
If your blood sugar levels are over 400 mg/dL, you could also be at risk for dehydration, says Dr. Spratt. During treatment, your doctors will give you insulin to stabilize your blood sugar levels and replace any fluids you might have lost through urination or vomiting, according to the U.S. National Library of Medicine.
Without treatment, diabetic ketoacidosis can lead to a diabetic coma and death, so if you have symptoms, seek immediate medical attention. Diabetes statistics show the condition was the seventh leading cause of death in the U.S. in 2017, according to the ADA, so it's important to get medical care right away.
Diabetes Numbers Warnings and Precautions
If your blood sugar reading is over 400 mg/dL, there is a chance that you have a false reading — particularly if you aren't experiencing any symptoms. For example, if there were food particles on your fingers or you used outdated or improperly stored test strips, you might have gotten an inaccurate reading.
If you re-test your levels and your blood sugar is indeed above 400 mg/dL, follow the advice your doctor has given you — whether that includes taking extra insulin or drinking more water — and contact him or her for further treatment advice.
Here's a chart to help you determine if your blood sugar levels are cause for concern:
What Your Blood Sugar Levels Mean
Normal (before a meal)
80 to 130 mg/dL
Normal (two hours after a meal)
Below 180 mg/dL
180 mg/dL or higher
Dangerously high (seek medical attention)
240 mg/dL or higher
- American Diabetes Association: "Hypoglycemia (Low Blood sugar)"
- American Diabetes Association: "DKA (Ketoacidosis) & Ketones"
- American Diabetes Association: "Understanding Blood Sugar and Control"
- American Diabetes Association: "The Big Picture: Checking Your Blood Glucose"
- U.S. National Library of Medicine: "Diabetic Ketoacidosis"
- American Diabetes Association: "Hyperglycemia (High Blood Glucose)"
- ADA: "Statistics About Diabetes"
- Mayo Clinic: "Hyperglycemia in diabetes"
- Cleveland Clinic: "Hyperglycemia"
- American Diabetes Association: Hyperosmolar Hyperglycemic Nonketotic Syndrome (HHNS)
- American Diabetes Association: DKA (Ketoacidosis) &amp; Ketones
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Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.