A fasting blood sugar of 100 mg/dL or higher is above the normal range, and 126 mg/dL or higher on two separate tests means diabetes. But “too high” depends on when you last ate, whether you’re pregnant, and whether you’re looking at a single reading or a long-term pattern. Here’s how to make sense of the numbers.
Normal vs. High Fasting Blood Sugar
Fasting blood sugar is measured after at least 8 hours without food or drink (other than water), typically first thing in the morning. The thresholds break down like this:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher
Prediabetes is a warning zone. Your body is already struggling to manage glucose, but the damage isn’t as advanced. Roughly 1 in 3 American adults falls into this range, and many don’t know it because there are usually no symptoms at this stage.
Blood Sugar After Meals
Your blood sugar naturally rises after eating, so the thresholds shift. For most people, a reading below 180 mg/dL two hours after the start of a meal is the target. Consistently landing above that level signals a problem with how your body processes glucose, even if your fasting numbers look fine.
Healthy people without diabetes typically peak well below 140 mg/dL after meals. If you’re monitoring at home and regularly seeing post-meal numbers above 180, that pattern is worth investigating with a blood test that captures the bigger picture.
A1C: The Long-Term Picture
A single blood sugar reading is a snapshot. The A1C test gives you a roughly three-month average by measuring how much glucose has attached to your red blood cells over their lifespan. The ranges:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
An A1C of 6.5% corresponds to an average blood sugar of about 140 mg/dL. This test is useful because it isn’t affected by what you ate yesterday or whether you were stressed during a single blood draw. It reveals the overall trend.
When Symptoms Start Showing Up
High blood sugar often causes no symptoms at all until levels climb above 180 to 200 mg/dL. That’s part of what makes it dangerous. You can walk around with prediabetes or early diabetes for years without feeling anything obvious.
Once blood sugar rises high enough, early symptoms include frequent urination, increased thirst, blurred vision, and unusual fatigue. These happen because your kidneys start working overtime to filter excess glucose, pulling extra water from your body in the process. The blurred vision comes from fluid shifts in the lenses of your eyes.
If levels continue climbing without treatment, more serious symptoms can develop: fruity-smelling breath, abdominal pain, nausea, vomiting, confusion, and shortness of breath. These are signs of a condition called ketoacidosis, where the body starts breaking down fat too quickly and produces toxic acids. This is a medical emergency.
Blood Sugar Levels That Are Emergencies
Not all high readings require a trip to the ER, but certain thresholds call for immediate action. If your blood sugar is 250 mg/dL or above, check it every 4 to 6 hours and test your urine for ketones. A reading that stays at 300 mg/dL or above is an emergency, and you should call 911 or go to the ER.
There’s also a condition that can push blood sugar above 600 mg/dL, most common in people with type 2 diabetes. At that level, the body can’t use glucose or fat for energy effectively, leading to severe dehydration, confusion, and loss of consciousness. This is life-threatening and requires emergency treatment.
Higher Thresholds During Pregnancy
Pregnancy changes how the body handles glucose, so the numbers shift. During a standard glucose screening (done between weeks 24 and 28), a blood sugar of 190 mg/dL or higher after drinking a glucose solution indicates gestational diabetes. A result below 140 mg/dL is typically considered normal.
If the initial screening is borderline, a longer follow-up test is done. Two or more readings above the expected range on that test confirm gestational diabetes. The thresholds during pregnancy are stricter than for the general population because even moderately elevated blood sugar can affect the baby’s growth and delivery.
Why Targets Vary by Age
Blood sugar goals aren’t one-size-fits-all. For older adults or anyone at higher risk of dangerous low blood sugar episodes, doctors often set more relaxed targets. The concern is that aggressively lowering blood sugar in someone who’s elderly or has other health conditions can cause drops that lead to falls, confusion, or heart problems.
For these individuals, spending at least 50% of the day with blood sugar between 70 and 180 mg/dL, and less than 10% of time above 250 mg/dL, is often considered a reasonable goal. Younger, otherwise healthy people with diabetes are generally held to tighter ranges.
What Chronic High Blood Sugar Does to Your Body
A single high reading after a big meal isn’t the issue. The real damage comes from blood sugar staying elevated over months and years. Excess glucose in the bloodstream acts like sandpaper on the inside of small blood vessels and nerves, gradually wearing them down.
The consequences hit multiple systems. Nerve damage can cause numbness, tingling, or pain in the hands and feet, eventually making daily tasks difficult. Kidney function declines as the tiny blood vessels that filter waste get damaged, potentially leading to chronic kidney disease. Reduced blood flow to the feet, combined with nerve damage, means small wounds heal slowly and infections become more likely. Digestive problems can develop when the nerves controlling the stomach are affected, slowing or stalling digestion. Sexual health is also impacted through nerve damage and reduced blood supply.
These complications develop gradually, often over 10 to 15 years of poorly controlled blood sugar. The key takeaway: a single number on a meter matters less than your pattern over time. Bringing your A1C down even by half a percentage point meaningfully reduces your risk for these complications.