Blood Sugar Over 400 mg/dL: Causes, Symptoms, Treatment, and Emergency Steps

A blood sugar level above 400 mg/dL (22.2 mmol/L) is a medical emergency until proven otherwise. It may signal severe hyperglycemia and can lead to life-threatening complications such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS). This guide explains the possible causes, warning signs, immediate treatment steps, and when to seek emergency medical care.

Written by: Dr. Albana Greca, MD, MMedSc, Family Physician and Medical Author.

Medically reviewed by: Dr. Ruden Cakoni, MD, Endocrinologist.

Last reviewed: July 2026.

Emergency Quick Answer

A blood sugar level over 400 mg/dL, or about 22.2 mmol/L, is severe hyperglycemia. Wash and dry your hands, check again with a new test strip, and contact your diabetes clinician or urgent-care service immediately for individualized instructions.

Go to the emergency department now for moderate or large ketones, vomiting, abdominal pain, deep or difficult breathing, fruity-smelling breath, confusion, extreme sleepiness, severe dehydration, fainting, chest pain, or inability to keep fluids down. A persistent reading over 400 mg/dL also requires urgent medical assessment even when symptoms seem mild.

A glucose result above 400 mg/dL is not simply “a little high.” It may occur with previously undiagnosed diabetes or with diabetes that is not receiving enough effective insulin or medication. It can progress to life-threatening dehydration, diabetic ketoacidosis (DKA), or hyperglycemic hyperosmolar state (HHS).

The reading needs attention whether it was measured fasting, after eating, after medication, or during illness. The cause may differ, but the immediate safety steps are similar.

What Does Blood Sugar Over 400 mg/dL Mean?

A reading above 400 mg/dL means there is a very high concentration of glucose in the bloodstream. At this level, the kidneys try to remove excess glucose through the urine, which pulls water and electrolytes from the body and can cause serious dehydration.

Blood sugar above 400 mg/dL does not by itself tell us whether a person has type 1 diabetes, type 2 diabetes, DKA, or HHS. Ketones, symptoms, hydration, blood chemistry, medical history, and laboratory tests are needed to determine the cause and severity.

Finding What it may mean Recommended action
One meter reading over 400 mg/dL Severe hyperglycemia or, less commonly, a testing error Wash and dry hands, repeat once, check ketones, and seek urgent medical advice.
Persistent reading over 400 mg/dL Insufficient effective insulin, illness, medication effect, or another urgent problem Urgent same-day assessment; emergency care may be necessary.
High glucose with ketones or serious symptoms Possible DKA or another hyperglycemic emergency Go to the emergency department immediately.

What to Do Immediately

  1. Wash and dry your hands, then test again. Sugar, fruit, or food residue can falsely raise a finger-stick result. Use an unexpired strip stored correctly.
  2. Check blood or urine ketones. Follow your personal sick-day plan. Moderate or large ketones require urgent medical advice and often emergency assessment.
  3. Use only your written correction plan. Do not guess an extra insulin dose, double a dose, or borrow another person’s insulin.
  4. Drink water if you can do so safely. Take small, frequent sips unless you have been told to restrict fluids because of heart or kidney disease.
  5. Do not exercise. Exercise can worsen hyperglycemia and ketone production when there is not enough effective insulin.
  6. Contact your diabetes team or urgent-care service now. Give them the glucose result, ketone level, symptoms, insulin or medication details, and any recent illness.

Do not stop insulin, diabetes medicine, antibiotics, steroids, heart medicine, or blood-pressure medicine without the prescriber’s instructions.

Why Can Fasting Blood Sugar Be Over 400 mg/dL?

During fasting, the liver normally releases glucose to supply the brain and other organs. Insulin limits this release. When the body lacks enough effective insulin, the liver may release too much glucose while the muscles and other tissues cannot use it properly.

Possible causes include:

  • Previously undiagnosed diabetes
  • Missed, delayed, or insufficient insulin or medication
  • Insulin pump, infusion-set, pen, syringe, or injection-site problems
  • Expired, frozen, overheated, or damaged insulin
  • Infection, fever, inflammation, surgery, pain, or another illness
  • Steroid treatment or another medication that raises glucose
  • Severe dehydration
  • A treatment plan that no longer meets the person’s needs

A late meal or the dawn phenomenon can contribute to high morning glucose, but they should not be used to dismiss a fasting level above 400 mg/dL.

Why Can Blood Sugar Rise Over 400 mg/dL After Eating?

Carbohydrates are broken down into glucose. In people without diabetes, insulin usually limits the after-meal rise. In diabetes, a very large meal, insufficient mealtime insulin, delayed insulin, incorrect carbohydrate counting, or a medication problem may cause glucose to remain dangerously high.

However, food is not always the main cause. Infection, dehydration, steroid use, pump failure, or severe insulin deficiency can produce high readings even when a person has eaten little.

For context, a common after-meal target for many nonpregnant adults with diabetes is below 180 mg/dL one to two hours after the start of a meal, although targets are individualized. A result above 400 mg/dL is far above an ordinary treatment target.

Can Medicines Cause Blood Sugar Over 400 mg/dL?

Some medicines can raise glucose or make diabetes more difficult to control. Corticosteroids are a common example. Certain antipsychotics, immunosuppressants, and other medicines may also contribute.

Do not stop a prescribed medicine suddenly. Some drugs, especially steroids, can be dangerous to stop without a supervised taper. Contact the prescribing clinician so the benefits, risks, glucose pattern, and possible treatment adjustments can be reviewed safely.

Symptoms of Blood Sugar Over 400 mg/dL

Symptoms may include:

  • Extreme thirst and dry mouth
  • Frequent urination
  • Blurred vision
  • Fatigue, weakness, or drowsiness
  • Headache
  • Nausea or vomiting
  • Abdominal pain
  • Rapid or deep breathing
  • Fruity-smelling breath
  • Confusion or reduced alertness
  • Very dry skin or other signs of dehydration

Some people, especially those with long-standing type 2 diabetes, may have fewer symptoms than expected. The absence of dramatic symptoms does not make a persistent reading above 400 mg/dL safe.

Diabetic Ketoacidosis and HHS

Diabetic ketoacidosis

DKA develops when the body does not have enough effective insulin and begins breaking down fat rapidly, producing acidic ketones. It is more common in type 1 diabetes, but it can also occur in type 2 diabetes, during pregnancy, and with certain diabetes medicines.

Warning signs include ketones, vomiting, abdominal pain, dehydration, fruity breath, rapid or deep breathing, and confusion. DKA can occur at glucose levels below 400 mg/dL, so the glucose number alone cannot rule it in or out.

Hyperglycemic hyperosmolar state

HHS usually occurs in people with type 2 diabetes and is often associated with profound dehydration and very high glucose. It may develop gradually during infection, missed treatment, or limited access to fluids.

HHS often involves glucose around 600 mg/dL or higher, but diagnosis is based on laboratory findings and mental status rather than one fixed home-meter threshold. Severe weakness, confusion, seizures, or loss of consciousness require emergency care.

How Is Severe Hyperglycemia Treated?

Treatment depends on the cause, symptoms, ketones, hydration, and laboratory findings. DKA and HHS are treated in a monitored medical setting with intravenous fluids, insulin, electrolyte replacement, frequent laboratory testing, and treatment of the trigger, such as infection or insulin interruption.

Do not attempt to reproduce hospital treatment at home. Potassium and other electrolytes may shift dangerously during treatment, which is one reason close monitoring is required.

Information to Prepare Before You Call

  • Current glucose result and recent readings
  • Whether the readings were fasting or after meals
  • Blood or urine ketone result
  • Symptoms and when they began
  • Insulin names, doses, and injection times
  • Diabetes medicines and any missed doses
  • Recent meals and fluid intake
  • Fever, infection, vomiting, diarrhea, surgery, injury, or pain
  • New medicines, especially steroids
  • Whether you use a pump or continuous glucose monitor
  • Pregnancy, kidney disease, heart disease, or fluid restrictions

A written blood sugar log helps the care team see whether the problem is new, repeated, meal-related, or associated with missed medication or illness.

How to Reduce the Risk of Another Reading Over 400 mg/dL

A blood sugar reading above 400 mg/dL often indicates that your diabetes management plan needs immediate attention. While emergency treatment focuses on lowering the current glucose level safely, preventing future episodes is equally important. The infographic below outlines practical steps that can reduce the risk of another severe high blood sugar episode and help you respond promptly if one occurs.

steps to prevent over 400 sugar reading

Balanced eating, weight management, smoking cessation, and regular activity support long-term diabetes care. They are not substitutes for urgent treatment during severe hyperglycemia. Do not exercise while glucose is extremely high with possible ketones or dehydration.

Important Safety Note

Call emergency services or go to the emergency department for vomiting, abdominal pain, deep or difficult breathing, fruity breath, confusion, fainting, severe weakness, moderate or large ketones, inability to drink, or signs of severe dehydration.

Children, pregnant people, anyone with type 1 diabetes, insulin-pump users, frail older adults, and people with kidney or heart disease need especially prompt medical guidance when glucose is above 400 mg/dL.

Doctor’s Note

A reading over 400 mg/dL is a warning that the current situation may be unsafe. Repeat the test correctly, check ketones, follow only your written correction plan, and obtain urgent professional guidance. Do not exercise, stop medication, or improvise extra insulin doses.

Most Asked Questions

Is blood sugar over 400 mg/dL always an emergency?

It always requires urgent attention. Emergency-department care is necessary when the level persists, ketones are moderate or large, serious symptoms are present, or the person cannot safely follow a clinician-directed correction plan.

Should I take extra insulin?

Use only the correction dose already prescribed for you. Do not guess a dose or repeat insulin too soon because insulin stacking can cause delayed severe hypoglycemia.

Should I drink water?

Small, frequent sips may help reduce dehydration if you can swallow safely and have no fluid restriction. Water does not replace insulin or emergency treatment.

Can I exercise to lower a reading over 400 mg/dL?

No. Exercise is not a safe home treatment at this level when ketones, dehydration, or insulin deficiency may be present.

Can DKA happen in type 2 diabetes?

Yes. DKA is more common in type 1 diabetes, but it can occur in type 2 diabetes, pregnancy, severe illness, and in association with some diabetes medicines.

Should I stop the medicine that may have caused the high glucose?

No. Contact the prescriber promptly. Stopping steroids or other prescribed medicines suddenly may be harmful.

Educational safety note: This article provides general diabetes education and does not replace emergency assessment, diagnosis, or personal treatment instructions. Do not start, stop, or change insulin, diabetes medicines, steroids, other prescriptions, diet, supplements, or exercise without guidance from your healthcare professional.

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