A “bad” glucose level is any reading outside the normal range, whether too high or too low. On a fasting blood test, anything at or above 100 mg/dL is considered elevated, and 126 mg/dL or higher indicates diabetes. On the low end, dropping below 70 mg/dL is considered hypoglycemia and can cause immediate symptoms. The full picture depends on when the test is taken, whether you’ve eaten, and what your body is going through.
Fasting Blood Sugar Ranges
A fasting blood sugar test, taken after at least eight hours without eating, is one of the most common ways to check glucose. The cutoffs break down clearly:
- Normal: 99 mg/dL or below
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or above
A single high reading doesn’t automatically mean diabetes. Doctors typically confirm a diagnosis by repeating the test or combining it with other measures. But if your fasting number consistently lands in the 100 to 125 range, your body is already struggling to regulate blood sugar efficiently. About 80% of people with prediabetes don’t know they have it, which is why that middle range matters so much.
After-Meal Blood Sugar
Blood sugar naturally rises after eating, so the thresholds are different from fasting numbers. Two hours after the start of a meal, a reading below 140 mg/dL is normal. Between 140 and 199 mg/dL falls into the prediabetes range, and 200 mg/dL or above points to diabetes.
For people already managing diabetes, the general target is to stay below 180 mg/dL two hours after eating. If you’re using a continuous glucose monitor, you can watch these spikes in real time. The goal for most adults with diabetes is to spend at least 70% of the day (roughly 17 hours) with glucose between 70 and 180 mg/dL.
A1C: The Bigger Picture
While individual readings capture a single moment, the A1C test reflects your average blood sugar over the previous two to three months. It measures how much sugar has attached to your red blood cells over their lifespan, giving a broader view of glucose control.
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or above
For people already diagnosed with diabetes, the typical target is 7% or less. An A1C of 8% or higher signals that blood sugar has been running too high over time, increasing the risk of complications like nerve damage, kidney problems, and vision loss. Even small improvements in A1C, on the order of half a percentage point, can meaningfully lower those risks.
When Low Blood Sugar Becomes Dangerous
Glucose levels don’t just become “bad” when they’re too high. Dropping below 70 mg/dL triggers a cascade of symptoms as your body releases adrenaline to compensate. You may feel shaky, sweaty, anxious, dizzy, or suddenly hungry. Your heart might race, and your thinking can get foggy. These symptoms hit quickly and demand an immediate response, typically 15 to 20 grams of fast-acting carbohydrates like juice, glucose tablets, or regular soda.
If blood sugar continues to fall without correction, the symptoms escalate. Severe hypoglycemia can cause blurred vision, coordination problems, confusion, seizures, and loss of consciousness. People who take insulin or certain oral diabetes medications are most at risk, especially if they skip a meal, exercise more than usual, or miscalculate a dose.
Emergency-Level High Readings
On the high end, a reading of 250 mg/dL or above is a red flag that requires closer monitoring. At that level, you should check blood sugar every four to six hours and test your urine for ketones, which are acids your body produces when it starts burning fat instead of sugar for energy. Elevated ketones can lead to diabetic ketoacidosis (DKA), a life-threatening condition.
If blood sugar stays at 300 mg/dL or above, that warrants emergency care. Symptoms of a diabetic emergency include a weak, rapid pulse, nausea, deep sighing breaths, confusion, flushed and dry skin, drowsiness, and a sweet or nail-polish-like odor on the breath. Loss of consciousness at this stage is a medical emergency requiring 911.
Glucose Ranges During Pregnancy
Pregnancy tightens the window for what counts as a safe glucose level. Screening for gestational diabetes typically happens between weeks 24 and 28. The initial screening is a glucose challenge test: you drink a sugary solution and have blood drawn an hour later. A result of 140 mg/dL or higher triggers a follow-up test.
The follow-up is a longer oral glucose tolerance test with blood draws at fasting, one hour, two hours, and three hours. If two or more of those readings come back high, gestational diabetes is diagnosed. For pregnant people using a continuous glucose monitor, the target range is tighter than for the general population: at least 70% of the day between 63 and 140 mg/dL.
What Your Numbers Actually Mean
Context changes everything about whether a glucose number is “bad.” A reading of 150 mg/dL right after a big meal is unremarkable. The same number after an overnight fast is a concern. Someone with well-controlled type 2 diabetes might celebrate staying under 180 after dinner, while the same reading in a person without diabetes would be unusual.
The pattern matters more than any single reading. One elevated fasting number could reflect a bad night of sleep, a stressful morning, or even the “dawn phenomenon,” where your liver dumps extra glucose in the early hours. Repeated high readings across multiple tests tell a much clearer story. If you’re checking at home and seeing numbers that consistently fall outside the ranges above, that’s the signal worth paying attention to.