A blood sugar reading between 250 and 400 mg/dL, or approximately 13.9–22.2 mmol/L, is very high. This is true whether the reading was measured after eating, before a meal, or after fasting overnight.
The meaning and urgency depend on whether you already have diabetes, how long the glucose has remained high, whether ketones are present, what treatment you use, and whether you have symptoms such as vomiting, dehydration, abdominal pain, confusion, or difficulty breathing.
For many nonpregnant adults with diabetes, commonly used glucose targets are approximately 80–130 mg/dL before meals and below 180 mg/dL about 1–2 hours after the beginning of a meal. Personal targets may be different, but 250–400 mg/dL is well above these general goals.
| Blood sugar | Equivalent in mmol/L | General interpretation |
|---|---|---|
| 250 mg/dL | 13.9 mmol/L | Very high; assess the cause and consider ketone testing. |
| 300 mg/dL | 16.7 mmol/L | Severely high; persistent readings can require emergency assessment. |
| 350 mg/dL | 19.4 mmol/L | Severely high and at increased risk of dehydration and metabolic complications. |
| 400 mg/dL | 22.2 mmol/L | Dangerously high; urgent medical evaluation is usually appropriate. |
A person may feel very unwell at these levels, but some people have few obvious symptoms. The absence of symptoms does not make a repeated reading of 250–400 mg/dL safe.
You can compare other readings with our normal blood sugar level chart or use the mg/dL and mmol/L blood sugar converter .
Blood sugar normally rises after eating because carbohydrates are broken down into glucose. Insulin then helps move glucose from the bloodstream into the body’s cells.
In diabetes, the body may not produce enough insulin, may not respond effectively to insulin, or may have both problems. As a result, glucose remains in the bloodstream and the after-meal rise becomes much higher or lasts longer than expected.
An after-meal reading in this range may occur after a large carbohydrate-containing meal, sugary drinks, missed medicine, insufficient insulin, illness, stress, steroid treatment, dehydration, or an insulin pump or infusion-set problem.
The timing also matters. A reading taken 20 minutes after eating is not interpreted in exactly the same way as one taken two hours after the meal. However, a result approaching 300–400 mg/dL remains concerning regardless of the exact timing.
For a broader explanation, see blood sugar levels after eating .
A fasting glucose test is performed after at least eight hours without food or caloric drinks. Water is normally allowed.
During fasting, the liver releases glucose to supply the brain and other organs with energy. Insulin usually keeps this release under control. In diabetes, insufficient insulin or insulin resistance can allow the liver to release more glucose than the body can manage.
A true fasting reading of 250–400 mg/dL is severely elevated. It may indicate untreated diabetes, a major deterioration in previously controlled diabetes, an infection or other illness, missed medication, insufficient insulin, steroid use, or another medical problem.
It should not be explained only by a late dinner or the dawn phenomenon without appropriate medical evaluation.
Ketones are produced when the body cannot use enough glucose for energy and begins breaking down fat. A dangerous buildup can cause diabetic ketoacidosis, also called DKA.
Check blood or urine ketones if:
Blood ketone meters can detect ketones earlier, while urine ketone strips are also commonly used. Follow the instructions provided with the test and the action levels given by your diabetes team.
DKA is more common in type 1 diabetes, but it can also occur in people with type 2 diabetes. People taking SGLT2 inhibitor medicines may occasionally develop DKA without an extremely high glucose reading. Feeling ill while taking one of these medicines requires prompt medical advice.
Possible symptoms include:
More serious symptoms such as vomiting, fruity-smelling breath, deep rapid breathing, confusion, severe drowsiness, fainting, or inability to drink can indicate DKA or another hyperglycemic emergency.
| Possible cause | What may be happening |
|---|---|
| Missed or insufficient insulin | The body does not have enough insulin to move glucose into cells. |
| Insulin pump or infusion-set problem | A blockage, leak, disconnection, empty reservoir, or spoiled insulin may interrupt delivery. |
| Missed diabetes tablets | The usual glucose-lowering effect may be reduced. |
| Large meal or sugary drink | A high carbohydrate load may overwhelm the available insulin response. |
| Illness or infection | Stress hormones can raise glucose and increase insulin requirements. |
| Corticosteroid treatment | Medicines such as prednisone can significantly raise blood sugar. |
| Emotional or physical stress | Adrenaline and cortisol may stimulate liver glucose release. |
| Dehydration | Fluid loss can make the glucose concentration rise further. |
| Previously undiagnosed diabetes | High glucose may be the first recognized sign of type 1 or type 2 diabetes. |
A result in this range is highly abnormal, but a home glucose meter is designed for monitoring and cannot formally diagnose diabetes.
Doctors use standardized laboratory tests such as:
| Laboratory test | Diabetes-range result |
|---|---|
| Fasting plasma glucose | 126 mg/dL or higher |
| HbA1c | 6.5% or higher |
| Two-hour oral glucose tolerance test | 200 mg/dL or higher |
| Random plasma glucose with classic symptoms | 200 mg/dL or higher |
When there are no classic symptoms or hyperglycemic crisis, doctors normally confirm an abnormal result with repeat laboratory testing or a second accepted diagnostic test.
If you have not previously been diagnosed with diabetes and obtain a confirmed home reading of 250–400 mg/dL, arrange urgent medical assessment. Do not wait several weeks for a routine checkup, particularly if you have thirst, frequent urination, weight loss, blurred vision, vomiting, or weakness.
Learn more about confirmation testing in our guide to blood tests for diabetes .
Treatment depends on the cause, the type of diabetes, current medicines, ketone results, hydration, kidney function, pregnancy status, and the presence of infection or another illness.
A medical review may include:
Severe dehydration, DKA, or another hyperglycemic crisis requires hospital treatment with monitored fluids, electrolytes, insulin, and treatment of the underlying cause.
Water can help replace fluid lost through frequent urination. Avoid regular soda, sweetened tea, energy drinks, and fruit juice while glucose is high, unless these are specifically needed to treat low blood sugar.
Extreme fasting is not a safe treatment for severe hyperglycemia. Follow your sick-day or meal plan and ask your healthcare provider what to eat if you feel nauseated or cannot tolerate normal meals.
Exercise often helps glucose management, but it can make glucose rise further when there is insufficient insulin and ketones are present. If glucose is above approximately 240–250 mg/dL, check ketones before exercising when your diabetes plan advises it.
Avoid strenuous exercise if ketones are positive, if you feel unwell, or if glucose is approaching 300–400 mg/dL. Follow your healthcare professional’s instructions instead.
For everyday meal planning after the immediate high-glucose problem has been addressed, see how carbohydrates and glycemic index affect blood sugar .
It is very high and should be taken seriously. Check the result again, follow your prescribed high-glucose plan, and consider ketone testing, especially if the level persists or you feel ill.
A single reading requires confirmation and assessment. Emergency care is recommended if glucose stays at 300 mg/dL or higher or you have vomiting, fruity breath, breathing difficulty, confusion, high ketones, severe dehydration, or multiple DKA symptoms.
Yes. DKA is more common in type 1 diabetes, but people with type 2 diabetes can also develop it, particularly during severe illness, insufficient insulin, dehydration, or treatment with certain medicines.
Water may help dehydration, but it cannot replace insulin or emergency treatment. A confirmed reading near 400 mg/dL usually requires urgent professional medical advice, especially when symptoms or ketones are present.
Use extra insulin only according to the correction factor or high-glucose plan prescribed for you. Guessing a dose or repeating doses too quickly can cause severe hypoglycemia.