Most high readings can wait for a phone call. A specific handful cannot. Here is the exact checklist emergency physicians, endocrinologists, and diabetes educators use to decide who needs a hospital bed tonight — and who needs a glass of water and a recheck in two hours.
Go to the ER if your blood glucose is above 250 mg/dL (13.9 mmol/L) with moderate-to-large ketones, above 400 mg/dL with any symptoms, or if you have vomiting you cannot stop, deep rapid breathing, fruity breath, confusion or extreme drowsiness, severe abdominal pain, or signs of severe dehydration. These signal diabetic ketoacidosis or hyperosmolar hyperglycemic state — both life-threatening and both treatable when caught early.[1]
- The ER checklist: 10 signs you should not wait on
- How high is too high? Reading the numbers
- DKA and HHS: the two hyperglycemic emergencies
- What to do in the minutes before help arrives
- Three mistakes that land people in the ICU
- What good high-blood-sugar care actually looks like
- When it is not the ER: urgent care, your care team, or home
- Situations that change the rules
- Frequently asked questions
The ER Checklist: 10 Signs You Should Not Wait On
If any single item on this list applies to you right now, stop reading and act on it. You do not need to have all ten. You do not need a "bad enough" number. One item is enough — the combination of a high glucose reading and any of the clinical signs below is what emergency physicians treat as a hyperglycemic crisis until proven otherwise.
If the person is confused, unresponsive, breathing deeply and rapidly, or cannot swallow safely, call emergency services. DKA and HHS can cause dangerous arrhythmias and cardiac arrest during transport, and paramedics can start IV fluids and give oxygen in the ambulance.
How High Is Too High? Reading the Numbers
A single number without context tells you very little. The same 280 mg/dL reading means something entirely different in a person with type 1 diabetes who has been vomiting for six hours versus someone who ate a large restaurant meal and forgot their metformin. What changes your next step is the number plus how you feel, plus whether ketones are present.
| Blood glucose | What it usually means | Your next step |
|---|---|---|
| 70–180 mg/dL (3.9–10.0 mmol/L) | Typical target range for most adults with diabetes | Continue routine monitoring and medications |
| 180–250 mg/dL (10.0–13.9 mmol/L) | Above target; common after meals or with stress and mild illness | Drink water, take medications as prescribed, recheck in 2–4 hours |
| 250–300 mg/dL (13.9–16.7 mmol/L) | Ketone production becomes possible | Check urine or blood ketones. Moderate/large ketones = ER |
| 300–400 mg/dL (16.7–22.2 mmol/L) | Significant hyperglycemia; dehydration developing | Contact your care team the same day; recheck every 2 hours |
| Above 400 mg/dL (22.2 mmol/L) | Severe hyperglycemia | Urgent contact with your care team; ER if any warning sign or you cannot reach anyone |
| Above 600 mg/dL (33.3 mmol/L) | Hyperosmolar range, especially in older adults with type 2 diabetes | Call 911. Do not drive yourself to the hospital |
| Any level plus moderate/large ketones plus vomiting | Diabetic ketoacidosis until proven otherwise | Emergency department now |
Two reads matter as much as one. A reading of 320 mg/dL that falls to 210 two hours after a correction dose is a different situation from the same 320 that climbs to 380 despite treatment. Persistence and direction of travel tell the real story.
Check ketones with a urine strip or a blood ketone meter whenever glucose is above 240 mg/dL during illness, or whenever you have nausea, vomiting, or abdominal pain — this is the threshold the American Diabetes Association uses in its sick-day guidance.[1] Blood ketone testing is more precise: readings at or above 1.5 mmol/L are generally treated as significant ketosis and warrant urgent evaluation, while 3.0 mmol/L and above represents severe ketoacidosis.
For context, roughly 38 million Americans live with diabetes, and hyperglycemic crises remain among the most preventable diabetes-related hospitalizations.[2] The people who come through them without lasting harm are almost always the ones who left the house four hours earlier than they wanted to.
DKA and HHS: The Two Hyperglycemic Emergencies
High blood sugar becomes an emergency through one of two distinct mechanisms, and knowing which one you are dealing with explains why the symptoms differ so much between patients.
Who: Most often type 1 diabetes; can occur in type 2 under severe stress.
Glucose: Usually above 250 mg/dL — but can be near-normal in euglycemic DKA.
Ketones: Moderate to large; the defining feature.
Typical labs: pH below 7.30, bicarbonate below 18 mEq/L.
Speed: Hours to 1–2 days.
Who: Older adults with type 2 diabetes; often nursing home residents or those with infection.
Glucose: Often above 600 mg/dL, sometimes over 1,000.
Ketones: Minimal or absent — there is enough insulin to block ketone formation.
Typical labs: pH above 7.30, osmolality above 320 mOsm/kg.
Speed: Days to weeks of gradual decline.
In DKA, insulin is so scarce that the body cannot move glucose into cells at all, so it burns fat instead — producing acidic ketone bodies that build up faster than the body can buffer them. In hyperosmolar hyperglycemic state (HHS), enough insulin remains to prevent ketone production, but not enough to keep glucose in check. Glucose rises for days or weeks, pulling water out of every cell in the body and concentrating the blood to a syrup-like consistency.
"DKA and HHS are the two ends of the same failure — one burns fat, the other drains water. Both are survivable if treated within hours and dangerous if left for days."
Clinical framing drawn from the ADA Standards of Care in Diabetes—2026
The prognosis gap between the two is real and worth understanding. With modern treatment, DKA mortality is under 1% in centers experienced with its management. Hyperosmolar hyperglycemic state still carries mortality several times higher — historically in the 5–10% range — because it strikes people who are older, often frail, and frequently already hospitalized or institutionalized for another illness before anyone checks a glucose.[1] That is precisely why "she just seems more confused and sleepy this week" in an 82-year-old with type 2 diabetes deserves a fingerstick, not reassurance.
A third pattern deserves its own mention. SGLT2 inhibitors — a widely prescribed class including empagliflozin, dapagliflozin, and canagliflozin — can trigger ketoacidosis at glucose levels that look almost normal, sometimes below 250 mg/dL and occasionally under 200. The FDA has issued a safety communication about this specifically, warning that ketoacidosis with these drugs can occur with only modestly elevated glucose, which delays recognition.[3] If you take one of these medications and develop nausea, vomiting, abdominal pain, or unusual fatigue, check ketones even if your glucose reading looks unremarkable — and say the drug name out loud when you arrive at the ER.
What to Do in the Minutes Before Help Arrives
The hours between deciding to get help and actually being evaluated are where outcomes are quietly decided. Work through these six steps in order.
Three Mistakes That Land People in the ICU
1. "I'll see if it comes down on its own." Waiting two or three more hours is the single most common reason a manageable case of early DKA becomes an ICU admission. High glucose that is not responding to a correction dose is not going to fix itself overnight.
2. Using a sugary drink to "stay hydrated." Juice, regular soda, and sports drinks are appropriate for treating low blood sugar — not high blood sugar. They add fuel to the fire. During illness with high glucose, the fluid should be sugar-free.
3. Taking a large insulin dose without monitoring afterward. Overshooting with insulin can produce severe hypoglycemia, and if you are vomiting or not eating, the timing of that drop is unpredictable. Correction doses should follow your written plan, and you should recheck glucose within two hours.
4. Skipping ketone testing because you "feel okay-ish." Ketones rise before you feel terrible. By the time nausea, vomiting, or heavy breathing starts, ketone levels are typically already substantial.
What Good High-Blood-Sugar Care Actually Looks Like
The patients who avoid emergency departments entirely are not the ones with the most stable diabetes — they are the ones with a written plan and the supplies to execute it at 2 a.m. A sick-day plan from your care team should specify your target glucose range during illness, how often to check, when to check ketones, what correction doses to use, which medications to continue or pause, and the specific number or symptom at which you should call or go in.[1]
Checking glucose every 3–4 hours during any illness, and every 2 hours if readings are climbing. Checking ketones whenever glucose is above 240 mg/dL or symptoms appear. Keeping unexpired urine ketone strips or a blood ketone meter at home at all times. Having a 24-hour number for your diabetes care team saved in your phone. Carrying medical identification that says you have diabetes and lists your medications. Knowing your own personal ER threshold before you need it — because during a crisis is not the time to be doing research.
One practical addition: write your emergency threshold on a card and tape it inside a kitchen cabinet. Something like "Call the ER if glucose is over 400, or over 250 with moderate ketones, or if I am vomiting and cannot keep water down." Decisions made under the fog of hyperglycemia are worse decisions, and a card removes the judgment call entirely.
When It Is Not the ER: Urgent Care, Your Care Team, or Home
An emergency department is the right destination for a small fraction of high readings. Plenty of 200s and even an occasional 300 without symptoms are not emergencies, and treating every elevated number as one leads to unnecessary exposure, long waits, and medical bills that discourage people from seeking care when it genuinely matters.
A high reading without any of the ten checklist items usually calls for a call to your care team, not a trip to the hospital. Post-meal spikes to 200–250 mg/dL, a high morning reading from dawn phenomenon, a missed dose, or a single elevated number during a cold are all situations that can typically be worked through by phone with a nurse or diabetes educator during office hours — and by an on-call endocrinologist or advice line after hours.
Urgent care occupies an awkward middle ground. These centers can check glucose, run basic labs, give fluids, and administer a dose of insulin, but most cannot run serial electrolyte panels, manage an insulin infusion, or admit a patient if things worsen. For a glucose in the 300s with mild dehydration and no ketones in a person who feels otherwise well, urgent care may be reasonable. For anything on the ten-item checklist, it is not — you will likely be transferred to an emergency department anyway, having lost an hour.
In the United Kingdom, the same logic maps onto NHS guidance: call 111 for urgent advice about a high reading without red-flag symptoms, and call 999 for vomiting, breathing difficulty, confusion, or a glucose reading in the hyperosmolar range.[5] The thresholds are similar; only the phone numbers change.
One rule cuts across all of this: if you are unsure, err toward being seen. No emergency clinician will think less of you for coming in with a 320 reading and a bad feeling. They will, however, see the consequences of waiting several times a week.
Situations That Change the Rules
Standard thresholds shift depending on who you are and what medications you take. Expand the situations below that apply to you.
You take an SGLT2 inhibitor (empagliflozin, dapagliflozin, canagliflozin)
Ketoacidosis can develop at glucose levels that look reassuring — sometimes under 200 mg/dL. This is called euglycemic DKA, and the FDA has issued a specific warning about it for this drug class.[3] If you have nausea, vomiting, abdominal pain, or unusual tiredness, check ketones regardless of your glucose reading, and tell the triage team you take one of these drugs.
SGLT2 inhibitors are commonly paused during acute illness, surgery, or prolonged fasting — but only under instructions from your prescriber.
You have type 1 diabetes
The threshold for concern is lower and the timeline is faster. DKA can develop within 6–12 hours, particularly with a failed insulin pump, an infected infusion site, or missed basal insulin. Vomiting in a person with type 1 diabetes should be treated as a potential emergency rather than a stomach bug.
Never stop basal insulin during illness, even when not eating.
You are over 65 with type 2 diabetes
Older adults are the group most likely to develop hyperosmolar hyperglycemic state, which builds over days rather than hours and often presents first as confusion, weakness, or a fall rather than as a dramatically high reading.[4] A change in mental status in an older adult with diabetes should prompt a fingerstick check immediately.
You are pregnant or postpartum
Pregnancy changes insulin requirements substantially, and infection, steroid use for fetal lung maturity, and postpartum hormonal shifts can all destabilize glucose control rapidly. Persistent readings above 200 mg/dL during pregnancy, or any vomiting with high glucose, should be evaluated urgently — call your obstetric or diabetes care team first if you have one, but do not delay care to reach them.
You use an insulin pump
A kinked cannula, an infusion site in scar tissue, or an empty reservoir can stop insulin delivery without any alarm. If glucose is above 250 mg/dL and rising, give a correction dose by injection rather than through the pump, change the entire infusion set, and check ketones. Pump users should always keep injectable basal insulin on hand as a backup.
You are taking corticosteroids or have an active infection
Steroids such as prednisone reliably raise glucose, often dramatically, especially in the afternoon and evening. Infections — urinary tract infections, pneumonia, cellulitis, dental abscesses — are among the most common DKA triggers. When a high reading is accompanied by fever, a productive cough, painful urination, or a red and swollen area of skin, the underlying infection often needs treatment alongside the glucose problem.
Frequently Asked Questions
Is a blood sugar of 300 an emergency?
Not automatically. A single reading of 300 mg/dL in someone who feels well, has no ketones, and can hydrate is usually a same-day call to the care team rather than an ER visit. It becomes an emergency when it comes with vomiting, abdominal pain, deep breathing, drowsiness, or moderate-to-large ketones — or when it fails to fall after a correction dose. Context decides, not the number alone.
Can high blood sugar actually kill you?
Yes, if it progresses to DKA or hyperosmolar hyperglycemic state and goes untreated. Both cause severe dehydration, electrolyte derangements, and acid-base disturbances that can lead to arrhythmias, cerebral edema, and cardiac arrest. With prompt treatment, DKA mortality is under 1%; untreated or treated late, it is a different story.[1]
What glucose level should trigger a 911 call?
Call 911 for any glucose reading above 600 mg/dL, or for any reading at all accompanied by confusion, unresponsiveness, deep rapid breathing, seizure, or an inability to swallow safely. In those situations, transport and pre-hospital treatment matter more than the exact number. Do not attempt to drive the person yourself.
Can DKA happen with a normal blood sugar?
Yes. Euglycemic DKA — ketoacidosis with glucose under 250 mg/dL and sometimes under 200 — occurs most often in people taking SGLT2 inhibitors, during pregnancy, after prolonged fasting, or with heavy alcohol use. This is why ketone testing matters even when the glucose number looks acceptable.[3]
Should I go to urgent care or the ER?
Urgent care can handle a high reading with mild dehydration and no red-flag symptoms — but most urgent care centers cannot manage an insulin infusion, run serial electrolyte panels, or admit you if things worsen. If anything on the ten-item checklist applies, go directly to an emergency department. A transfer from urgent care adds time you may not have.
What happens once I get to the ER for high blood sugar?
Expect immediate glucose and ketone testing, blood draws for electrolytes and kidney function, an ECG, and IV fluid resuscitation. If DKA is confirmed, treatment typically follows a protocol: isotonic fluids first, then a continuous insulin infusion, with potassium added once levels are known. Glucose usually normalizes before the acidosis fully resolves, so patients often stay in hospital until the bicarbonate and anion gap normalize too — typically 24–48 hours.
- Go to the ER for glucose above 250 mg/dL with moderate-to-large ketones, above 400 mg/dL with any symptoms, or any reading accompanied by vomiting, deep rapid breathing, fruity breath, confusion, severe abdominal pain, or severe dehydration.
- DKA and hyperosmolar hyperglycemic state are the two hyperglycemic emergencies; DKA develops in hours, HHS over days, and HHS carries substantially higher mortality.
- Check ketones whenever glucose exceeds 240 mg/dL during illness — the ADA sick-day threshold — and any time you feel nauseated or have abdominal pain.
- SGLT2 inhibitors can cause ketoacidosis at near-normal glucose levels, so check ketones even when the reading looks acceptable.
- Never stop basal insulin during illness, even when you are not eating — lack of basal insulin is what drives ketone production.
- Call 911 rather than driving if the person is confused, drowsy, breathing deeply, or cannot swallow.
- American Diabetes Association. Standards of Care in Diabetes—2026 — diagnostic criteria and management of diabetic ketoacidosis and hyperosmolar hyperglycemic state; sick-day ketone monitoring thresholds. diabetes.org
- Centers for Disease Control and Prevention. National Diabetes Statistics Report — diabetes prevalence and diabetes-related hospitalization data. cdc.gov
- U.S. Food and Drug Administration. Drug Safety Communication: SGLT2 inhibitors and risk of ketoacidosis, including euglycemic presentations. fda.gov
- MedlinePlus (U.S. National Library of Medicine). Diabetic Ketoacidosis and Hyperglycemia — symptom and complication reference. medlineplus.gov
- National Health Service (UK). Diabetes and sick day rules; when to call 111 or 999. nhs.uk
