A blood sugar reading over 500 mg/dL is a medical emergency. At this level, your body is under severe metabolic stress, and two life-threatening conditions can develop: diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS). Both require emergency treatment, and delays can lead to organ damage, coma, or death.
What Happens Inside Your Body
When glucose in your blood climbs past 500 mg/dL, the concentration of your blood rises dramatically. This creates an osmotic pull that draws water out of your cells and into the bloodstream, essentially dehydrating your tissues from the inside out. Your kidneys try to flush the excess sugar by producing large amounts of urine, which accelerates fluid loss even further. The result is a vicious cycle: the more dehydrated you become, the more concentrated your blood gets, and the higher your sugar climbs.
This severe dehydration thickens your blood and can damage the lining of blood vessels, increasing the risk of dangerous blood clots. Your kidneys take a direct hit. In a large Danish study of patients hospitalized with HHS, 65% had acute kidney injury at the time of admission. The kidneys simply can’t keep up when forced to filter that much glucose while running low on fluid.
Two Emergencies That Can Develop
Blood sugar over 500 mg/dL puts you in the range for two distinct crises, and they look and feel different.
Diabetic ketoacidosis (DKA) happens when your body has so little usable insulin that cells start burning fat for energy instead of glucose. This produces acidic byproducts called ketones that build up in the blood, making it dangerously acidic. DKA is more common in people with type 1 diabetes but can occur in type 2 as well. Blood sugar in DKA typically ranges from 200 to 600 mg/dL, and the real danger comes from the acid buildup rather than the glucose level alone.
Hyperosmolar hyperglycemic state (HHS) is the more deadly of the two. It typically involves blood sugar above 600 mg/dL and extreme dehydration, but without significant ketone production. HHS develops more slowly, often over days or weeks, and tends to affect older adults with type 2 diabetes. Because it creeps up gradually, people often don’t seek help until they’re severely ill. The in-hospital mortality rate for HHS is approximately 17%, roughly ten times higher than DKA.
Some people develop a combination of both conditions at the same time, with a mortality rate around 9%.
Warning Signs to Recognize
The symptoms of a hyperglycemic crisis build on each other. Early signs include excessive thirst, frequent urination, weakness, and blurred vision. As the situation worsens, the symptoms become more alarming.
In DKA, your breathing may become unusually deep and rapid as your body tries to blow off excess acid through your lungs. This pattern, called Kussmaul breathing, is one of the hallmark signs of the condition. Your breath may also take on a fruity or acetone-like smell from the ketones. Nausea, vomiting, and abdominal pain are common. About half of people with DKA arrive at the hospital lethargic or in a stupor, though fewer than 25% are fully unconscious.
In HHS, the neurological symptoms tend to be more severe. Confusion progresses to a declining level of consciousness, and seizures or stroke-like symptoms can occur. Many people with HHS present with a history of progressive mental decline over several days, along with extreme weakness and dehydration.
The Risk to Your Brain
Extremely high blood sugar poses a direct threat to brain function. The severe dehydration and high blood concentration alter how fluid moves in and out of brain cells, which can cause confusion, seizures, and coma. Both DKA and HHS can present with sudden, severe neurological impairment.
There’s also a risk during treatment. If blood sugar and fluid levels are corrected too quickly, the rapid shift in blood concentration can cause fluid to rush into brain cells faster than they can adapt. This leads to brain swelling, a rare but serious complication that occurs in roughly 0.7% to 1.0% of cases. Symptoms include worsening headache, altered consciousness, seizures, and changes in pupil size. This is one reason why emergency teams lower blood sugar gradually rather than all at once.
What to Do if Your Meter Reads Over 500
If your glucose meter shows a reading above 500 mg/dL, or displays “HI” (which typically means the reading exceeds the meter’s upper limit), recheck immediately. Wash your hands first, use a fresh test strip that isn’t expired, and test again. A false high can happen from residue on your fingers or degraded strips.
If the second reading confirms the number, this is not something to manage at home on your own. Call your doctor or endocrinologist immediately. If you can’t reach them, or if you’re experiencing any of the symptoms described above, go to the emergency room. While waiting, you can drink water to help with dehydration, but do not exercise. Exercise with very high blood sugar and ketones present can push levels even higher.
If you have urine ketone test strips at home, use them. Positive ketones combined with blood sugar over 500 strongly suggest DKA is developing. Even without ketones, blood sugar at this level can indicate the early stages of HHS.
What Happens at the Hospital
Emergency treatment focuses on three things: rehydration, lowering blood sugar, and correcting electrolyte imbalances. You’ll receive fluids through an IV to replace the liters of water your body has lost. Insulin is given through a continuous IV drip, carefully titrated to bring blood sugar down at a controlled pace. Dropping it too fast increases the risk of brain swelling, so medical teams monitor levels closely and adjust the rate.
Your potassium, sodium, and other electrolytes will be checked repeatedly, because insulin and fluid replacement shift these levels rapidly and dangerous imbalances can develop. Kidney function is monitored throughout, since acute kidney injury is common on admission. Most kidney damage at this stage is reversible with proper fluid replacement, but it depends on how long the crisis went untreated.
Hospital stays for hyperglycemic emergencies vary. DKA often resolves within 24 to 48 hours with treatment, while HHS can take longer because the dehydration is typically more severe. Recovery depends heavily on how quickly treatment begins, which is why recognizing the signs early matters so much.
Why Blood Sugar Reaches This Level
Blood sugar doesn’t usually jump to 500 mg/dL without a triggering event. The most common causes include missed insulin doses, infections (especially urinary tract infections and pneumonia), new-onset diabetes that hasn’t been diagnosed yet, and other acute illnesses that spike stress hormones and drive glucose up. Certain medications, particularly corticosteroids, can also push levels dangerously high.
For people with type 2 diabetes, HHS often develops during an illness when fluid intake drops and blood sugar creeps up unnoticed over days. Older adults living alone are particularly vulnerable because the early confusion can prevent them from recognizing the problem or seeking help. If you have diabetes and become sick with a fever, vomiting, or any illness that limits your ability to eat and drink normally, checking your blood sugar more frequently can catch a dangerous rise before it reaches crisis territory.