Written by: Dr. Albana Greca Sejdini, MD, MMedSc • Medically reviewed by: Dr. Ruden Cakoni, MD, Endocrinologist Last medically reviewed: August 2026
One of the most common questions patients ask me is, “At what blood sugar level should I become worried?” It is an important question, but there is not one glucose number that separates safe from dangerous in every person.
Very low glucose can interfere with brain function, while very high glucose can lead to severe dehydration and medical emergencies such as diabetic ketoacidosis (DKA) or hyperosmolar hyperglycemic state (HHS). I therefore look at the glucose number together with symptoms, ketones, illness, medications, hydration and how quickly the level is changing.
If you are trying to understand an ordinary reading rather than an emergency, start with our normal blood sugar level chart or compare your result with our blood sugar levels by time of day. This page focuses specifically on readings and symptoms that may require urgent action.
A glucose level below 70 mg/dL (3.9 mmol/L) is low and should be treated promptly. A level below 54 mg/dL (3.0 mmol/L) is clinically significant hypoglycemia. Severe hypoglycemia becomes an emergency when a person cannot safely treat themselves, regardless of the exact glucose number.
There is no single high-glucose number that defines every emergency. Persistent readings around 250–300 mg/dL deserve attention, especially during illness or when ketones or symptoms are present. Readings above 400 mg/dL are extremely high and need prompt assessment if they do not improve or serious symptoms occur.
A glucose of 600 mg/dL (33.3 mmol/L) or higher is one criterion used when doctors evaluate for HHS. However, DKA can occur at much lower glucose levels, sometimes even below 200 mg/dL.
When blood sugar is abnormal, I do not want my patients to watch only the meter. Certain symptoms tell us that the brain, circulation, hydration or body's acid-base balance may already be affected.
This is especially important with diabetic ketoacidosis, severe hypoglycemia and HHS. If severe warning signs are present, do not keep waiting for the glucose to reach a particular number.
Seek emergency medical care immediately if a person develops:
Chest pain, severe shortness of breath, face drooping, arm weakness, trouble speaking or other signs of heart attack or stroke also require emergency care regardless of the glucose reading.
The following chart helps you understand how different glucose levels are generally interpreted. It is deliberately not called a “safe versus dangerous” chart, because the same glucose value may have a different meaning depending on symptoms, timing, illness, ketones and whether you already have diabetes.
For comparison, ordinary fasting and after-meal readings are explained separately in our normal fasting blood sugar guide and blood sugar after eating chart. The ranges below focus on levels requiring increased attention.
| Glucose | mmol/L | What it may mean | General action |
|---|---|---|---|
| <54 mg/dL | <3.0 | Level 2, clinically significant hypoglycemia | Treat immediately; get help if the person cannot safely self-treat |
| 54–69 mg/dL | 3.0–3.8 | Level 1 hypoglycemia | Treat promptly and recheck |
| >180 mg/dL | >10.0 | Above the usual post-meal target for many adults with diabetes | Look at the pattern; repeated highs may require treatment review |
| About 250–300 mg/dL | 13.9–16.7 | Very high; more concerning with illness, missed insulin, dehydration or ketones | Follow your sick-day/correction plan and seek advice if it persists or symptoms occur |
| >400 mg/dL | >22.2 | Extremely high glucose with increased risk of dehydration and metabolic deterioration | Prompt medical advice; emergency evaluation if significant symptoms or ketones occur |
| ≥600 mg/dL | ≥33.3 | Meets the glucose criterion used when evaluating for HHS | Urgent hospital evaluation is required |
The key message is that you should not wait for glucose to reach 300, 400 or 600 mg/dL when serious symptoms are already present. DKA can occur at substantially lower glucose levels, and severe hypoglycemia is determined partly by the person's ability to function and treat themselves.
Many patients naturally want to enter a glucose number and receive a simple answer. A checker can help show whether the reading falls into a low, usual, high or very high range, but it cannot determine whether someone is medically safe from the glucose number alone.
Before using any calculator or checker, remember that the timing of the reading matters. A result before breakfast is interpreted differently from one taken after lunch. Our blood sugar levels by time of day guide explains these differences in more detail.
High blood sugar, or hyperglycemia, may cause thirst, frequent urination, fatigue and blurred vision. When glucose remains very high, the kidneys try to remove excess glucose through urine. Water is lost with it, which can gradually lead to dehydration.
A single high reading does not always mean an emergency. It is the combination of how high the glucose is, how long it stays high, symptoms and the reason it is rising that determines the level of concern. For ordinary after-meal elevations, see our guide to high blood sugar after eating.
A glucose level around 250–300 mg/dL (13.9–16.7 mmol/L) is very high. It does not automatically mean that DKA is present, but it deserves attention—particularly in people with type 1 diabetes, during illness, after missed insulin or when ketones are present.
Warning symptoms that make the situation more urgent include:
If you use insulin, follow the sick-day and correction instructions given by your own diabetes team. Do not improvise repeated correction doses. Our insulin bolus and correction calculator is an educational tool for people who already know their prescribed correction factor; it is not an emergency-treatment calculator.
A confirmed glucose above 400 mg/dL (22.2 mmol/L) is extremely high. At this level, dehydration can worsen quickly, especially if the person is urinating frequently, vomiting, unable to drink normally or fighting an infection.
Seek urgent medical assessment when very high glucose does not improve according to your prescribed plan or occurs with ketones, vomiting, abdominal pain, rapid/deep breathing, severe dehydration, confusion or extreme drowsiness.
Low blood sugar deserves special attention because the brain depends heavily on glucose. A person may initially feel hungry, shaky or sweaty, but as glucose falls further, thinking, coordination and consciousness can become impaired.
If you want a more detailed explanation of these numbers, see our guides to blood sugar between 45 and 69 mg/dL and blood sugar below 54 mg/dL.
Current diabetes guidance divides hypoglycemia into three clinically useful levels:
| Level | Definition | Why it matters |
|---|---|---|
| Level 1 | Below 70 but at least 54 mg/dL | Low glucose requiring prompt treatment |
| Level 2 | Below 54 mg/dL (3.0 mmol/L) | Clinically significant hypoglycemia requiring immediate action |
| Level 3 | Severe event requiring another person's assistance | Medical emergency defined by severity, not a specific glucose number |
Repeated episodes of low glucose should not simply be accepted as part of diabetes treatment. Our page on causes of low blood glucose explains how insulin, medications, meals, exercise, alcohol, kidney function and other factors may contribute.
If glucose is below 70 mg/dL and the person is awake and able to swallow safely, the goal is to raise glucose promptly with fast-acting carbohydrate and then confirm that it has recovered.
A commonly used approach for most adults is the 15–15 rule:
Very abnormal glucose can lead to several different medical emergencies, and they do not all occur at the same glucose level. Understanding the difference between DKA, HHS and severe hypoglycemia helps explain why symptoms sometimes matter more than the meter reading.
These conditions require medical evaluation because treatment involves much more than lowering or raising glucose. Doctors may also need to correct dehydration, electrolyte problems, ketones and abnormal acidity and identify the illness or treatment problem that triggered the emergency.
Diabetic ketoacidosis occurs when the body does not have enough effective insulin. The body begins breaking down fat for energy, producing ketones that can accumulate and make the blood too acidic.
Common warning signs include nausea, repeated vomiting, abdominal pain, fruity-smelling breath, rapid/deep breathing, extreme fatigue, dehydration and confusion.
This is particularly important during illness, pregnancy, prolonged fasting and with SGLT2 inhibitor medicines. For a fuller explanation, read diabetic ketoacidosis symptoms, causes and emergency treatment.
Hyperosmolar hyperglycemic state usually develops differently from DKA. Glucose becomes extremely high, large amounts of water are lost through the urine, and severe dehydration and hyperosmolarity develop.
Possible signs include extreme thirst, frequent urination, dry mouth, marked weakness, confusion, increasing drowsiness, seizures and loss of consciousness.
A plasma glucose of 600 mg/dL (33.3 mmol/L) or higher is one criterion used when doctors evaluate for HHS, but glucose alone does not establish the diagnosis. Suspected HHS requires immediate hospital evaluation.
The term diabetic coma describes severe loss of consciousness related to a diabetes emergency rather than one particular glucose level. It can occur with severe hypoglycemia, DKA, HHS or another serious medical problem.
If someone with diabetes becomes extremely drowsy, confused, unresponsive, has a seizure or loses consciousness, do not wait for another glucose reading. Call emergency services. You can read more about diabetic coma warning signs and causes.
Ketones are produced when the body uses fat for energy. Small amounts may occur during fasting or reduced carbohydrate intake, so detecting any ketones does not automatically mean that someone has DKA.
In a person with diabetes, however, moderate or high urine ketones or significantly elevated blood ketones become more concerning when they occur with illness, missed insulin, persistent hyperglycemia or symptoms such as vomiting and abdominal pain.
If you are at risk for DKA, your sick-day plan should explain when and how to check ketones. Do not wait for glucose to become extremely high if DKA symptoms are developing.
If glucose is very high but you are awake, able to drink normally and do not have emergency symptoms, the first step is to follow the plan already provided by your diabetes team rather than improvising additional medication.
Never repeatedly take extra insulin unless this is part of a correction plan prescribed specifically for you.
Patients sometimes delay seeking help because they are waiting for a particular number to appear on the meter. I would rather you judge an emergency by the whole clinical situation.
Inability to drink, repeated vomiting, abnormal breathing, significant ketones and changes in consciousness may signal serious metabolic illness even when the glucose is below a number you expected to be “dangerous.”
Seek emergency assessment for:
Do not delay emergency care because glucose appears “not high enough.” Symptoms and the person's overall condition can be more important than the number alone.
A glucose emergency usually has a cause. Finding that trigger matters because simply correcting the glucose without addressing the underlying problem can allow the situation to happen again.
High glucose may be related to illness, infection, missed insulin, steroid medicines, dehydration or progression of diabetes. Low glucose may follow excessive insulin, certain diabetes medicines, skipped meals, alcohol or unexpected physical activity.
If low readings are the problem, see why blood glucose can become too low. If your concern is repeated morning highs rather than an acute emergency, our guide to high blood sugar in the morning explains dawn phenomenon, sleep, medication timing and other common causes.
Not every diabetes emergency can be prevented, but preparation can greatly reduce risk. I encourage my patients to create a plan for both high and low glucose before they become ill or find themselves dealing with an unexpected reading.
A useful emergency plan should cover glucose monitoring, medication, meals and fluids, ketone testing, hypoglycemia treatment and when another person or medical team should become involved.
For routine glucose management outside an emergency, see our blood sugar control guide.
When patients ask me, “What blood sugar number is dangerous?” I explain that I never look at the glucose value alone. I look at the number, symptoms, ketones, illness, medications, hydration and the direction glucose is moving. A very abnormal glucose deserves attention, but waiting for a particular number before asking for help can also be dangerous. If you are vomiting, confused, severely dehydrated, having difficulty breathing, having a seizure or unable to safely treat yourself, seek urgent medical care.
These are some of the questions patients most often ask after seeing a glucose reading much higher or lower than expected. Each answer describes a general level of concern; symptoms and your personal diabetes plan can change what you should do.
A glucose of 250 mg/dL (13.9 mmol/L) is very high but is not automatically an emergency by itself. It becomes more concerning if it persists, occurs during illness, follows missed insulin or is accompanied by ketones, vomiting, abdominal pain, dehydration or breathing changes.
A glucose around 300 mg/dL (16.7 mmol/L) is very high and needs attention. Follow your prescribed sick-day or correction plan and seek medical advice if it does not improve. DKA symptoms, significant ketones, dehydration or confusion require urgent assessment.
A glucose around 400 mg/dL (22.2 mmol/L) is extremely high and warrants prompt attention. Emergency evaluation is especially important if it does not improve or occurs with ketones, vomiting, dehydration, confusion or breathing difficulty.
No. A plasma glucose of 600 mg/dL or higher is one diagnostic criterion for HHS, but additional laboratory findings are required. A glucose at this level nevertheless warrants urgent hospital evaluation.
Yes. DKA can occur around 200 mg/dL and sometimes below 200 mg/dL. This is especially important with SGLT2 inhibitors, pregnancy, prolonged fasting and certain illnesses.
Glucose below 54 mg/dL (3.0 mmol/L) is clinically significant Level 2 hypoglycemia. Severe Level 3 hypoglycemia is defined by needing another person's assistance, regardless of the glucose number.
Yes. Some people have limited warning symptoms even when glucose is very high or low. Monitoring is particularly important for people taking insulin or medicines that can cause hypoglycemia.
For more detail about specific glucose situations and emergencies, these pages expand on the topics discussed above:
If your glucose is not currently in an emergency range, these broader guides can help you understand everyday readings and patterns:
This article is designed to help you recognize potentially dangerous glucose patterns and understand when professional or emergency medical assessment may be needed. It does not diagnose DKA, HHS, hypoglycemia or another medical emergency.
Glucose readings should be interpreted together with symptoms, ketones, medications, illness, hydration and your individual diabetes plan. Do not take repeated extra doses of insulin or change diabetes medication because of information on this page unless your healthcare professional has given you specific instructions.
The medical thresholds and emergency guidance on this page are based primarily on current diabetes standards and clinical guidance on hypoglycemia and hyperglycemic emergencies.