What Is Blood Sugar? Levels, Tests & What They Mean

Written by Syed Azmal Mahmood MD (Endocrinology), FCPS, MACP, MACE

Last updated September 22, 2026

Blood sugar, also called blood glucose, is the amount of glucose circulating in your blood. It’s your body’s main source of energy.

Glucose comes largely from the food you eat. A hormone called insulin helps move it from your bloodstream into your cells. Blood sugar naturally rises and falls throughout the day, but levels that are unusually high or low over time can signal a problem.

Testing helps identify prediabetes and diabetes early, often before symptoms appear. This guide explains what blood sugar is, what the common tests measure, and how to make sense of your results.

What Is Blood Sugar?

Blood sugar is simply the glucose in your bloodstream at any given moment. Your body works hard to keep this number within a fairly narrow range.

What Does Blood Sugar Do?

Glucose is one of your body’s most important energy sources. Carbohydrates in food — bread, fruit, rice, milk, and sweets — break down into glucose during digestion.

That glucose enters your bloodstream and travels to your cells for fuel. Your body tightly regulates this process so you have steady energy without dangerous swings.

What Is Insulin?

Insulin is a hormone made by the pancreas. It acts like a key, unlocking your cells so glucose can move out of the blood and into them for energy.

When this process doesn’t work well — a condition called insulin resistance — glucose builds up in the bloodstream instead. Over time, this can lead to prediabetes or type 2 diabetes.

What Causes Blood Sugar to Rise and Fall?

Many everyday factors influence your glucose levels, sometimes more than you’d expect.

Factors that can raise blood sugar:

• Eating carbohydrate-containing foods
• Stress
• Illness or infection
• Certain medications
• Reduced physical activity

Factors that can lower blood sugar:

• Physical activity
• Skipping or delaying meals
• Diabetes medications or insulin
• Alcohol, in certain situations

The CDC notes that food choices, medications, and physical activity all play a role in day-to-day blood sugar changes.

What Are Normal Blood Sugar Levels?

This section covers diagnostic ranges — the numbers doctors use to identify prediabetes and diabetes. These are different from personal treatment targets, which are covered in the next section.

These thresholds come from the American Diabetes Association and reflect commonly used diagnostic criteria.

A single abnormal result usually doesn’t confirm a diagnosis on its own. According to the ADA, most test results need to be confirmed on a second day, unless blood glucose is very high or classic symptoms are present alongside a positive test.

What Is a Normal Blood Sugar Level for Someone With Diabetes?

If you already have diabetes, the diagnostic ranges above aren’t your personal target. Treatment goals are individualized, based on your health history and care plan.

For many nonpregnant adults with diabetes, common targets include:

• Before meals: 80–130 mg/dL
• 1–2 hours after the start of a meal: below 180 mg/dL

Individual targets can differ based on age, other health conditions, pregnancy, medications, and your overall diabetes management plan. The American Diabetes Association notes that targets should be personalized and discussed with your care team, since factors like disease duration and hypoglycemia risk can shift what’s appropriate for you.

Key distinction: “Normal for diagnosis” is not the same as “target for someone managing diabetes.” Don’t apply diagnostic thresholds to your own treatment goals without asking your doctor first.

What Are the Different Blood Sugar Tests?

Several tests measure blood sugar in different ways. Here’s an overview of each.

Fasting Blood Sugar Test

This test is usually done after at least 8 hours without food, often first thing in the morning. It’s commonly used to identify prediabetes and diabetes, using the ranges in the table above.

A1C Test

The A1C test doesn’t require fasting. It measures your average blood glucose over roughly the past 2–3 months by checking how much glucose has attached to your red blood cells.

It’s used both to diagnose prediabetes and diabetes and to monitor how well an existing diabetes management plan is working. The ADA identifies 5.7–6.4% as the prediabetes range and 6.5% or higher as the diabetes range.

Oral Glucose Tolerance Test

For this test, you fast beforehand, then drink a glucose-containing beverage. Your blood glucose is measured afterward, typically at the 2-hour mark, which is commonly used for diagnosis.

Random Blood Sugar Test

This test can be done at any time, with no fasting required. A result of 200 mg/dL or higher can point to diabetes, especially when combined with symptoms and other clinical context.

Continuous Glucose Monitor

A continuous glucose monitor (CGM) uses a small sensor placed under the skin to track glucose at frequent intervals throughout the day and night. This gives a fuller picture than a single fingerstick reading.

CGMs are especially useful for people who use insulin, since they help reveal patterns and trends over time. The NIDDK describes CGMs as tools that check glucose levels automatically, often every few minutes, without repeated fingersticks.

What Does a Blood Sugar Test Result Mean?

Numbers alone don’t tell the full story. Symptoms and context matter too.

Low Blood Sugar

For people with diabetes, a reading below 70 mg/dL is generally considered low. Possible symptoms include:

• Shaking
• Sweating
• Hunger
• Dizziness
• Confusion
• Fast heartbeat
• Weakness

The NIDDK explains that low blood sugar, or hypoglycemia, can happen quickly and needs prompt treatment, especially in people using insulin or certain diabetes medications.

High Blood Sugar

Persistent or significantly elevated glucose can point to poor glucose control or diabetes, depending on the full picture. Possible symptoms include:

• Increased thirst
• Frequent urination
• Fatigue
• Blurred vision

If you’re noticing these symptoms and haven’t been diagnosed, our guide to diabetes symptoms covers what else to watch for. Symptoms and test results should always be interpreted together, not in isolation.

What Can Affect Your Blood Sugar Levels?

Day-to-day fluctuations are normal. Common influences include:

• Food and carbohydrate intake
• Physical activity
• Stress
• Sleep
• Illness or infection
• Diabetes medications
• Insulin
• Alcohol
• Hormonal changes
• Timing of meals
• Hydration

If your numbers seem to swing for no clear reason, tracking these factors alongside your readings can help you and your care team spot patterns.

How Is Blood Sugar Tested at Home?

Home testing with a blood glucose meter follows a simple process:

1. Wash and dry your hands.
2. Prepare the glucose meter and test strip.
3. Use the lancet according to your device’s instructions.
4. Apply the blood sample to the test strip.
5. Read and record the result.
6. Track patterns over time.

The CDC recommends washing and drying your hands before testing and keeping a record of results so patterns can be reviewed with your healthcare team.

Safety note: Never share lancets or blood glucose monitoring equipment, even with family members.

When Should You Check Your Blood Sugar?

Testing frequency depends on several factors:

• Type of diabetes
• Medications
• Insulin use
• Risk of hypoglycemia
• Your individual treatment plan

Common testing times include:

• On waking
• Before meals
• Two hours after meals
• At bedtime

The right schedule for you should be set by your healthcare team, not a general guideline. Some people test several times a day; others test far less often.

How Can You Manage Blood Sugar?

Management works best as a combination of habits rather than any single change.

Eat Balanced Meals

Focus on non-starchy vegetables, fiber-rich carbohydrates, protein, healthy fats, and appropriate portions. Our diabetes diet guide covers this in more detail, including practical meal ideas.

Stay Physically Active

Movement helps your body use insulin more effectively. See the benefits of exercise for more on how activity supports overall health, not just blood sugar.

Take Medications as Prescribed

Never adjust your dose or stop a medication without talking to your doctor first, even if your numbers look good for a while.

Monitor Patterns

Bring recurring highs or lows to your healthcare team rather than trying to interpret them alone. Patterns over days or weeks are more useful than any single reading.

Prioritize Sleep and Stress Management

Poor sleep and high stress can both push blood sugar higher. Our guides on how to improve sleep and how to reduce stress offer practical starting points.

Blood Sugar vs. A1C: What’s the Difference?

These two measurements answer different questions.

Think of a single glucose reading as a snapshot and A1C as a slower-moving average. Both matter, but they tell you different things.

When Should You Talk to a Doctor About Blood Sugar?

Reach out to your care team if you notice:

• Repeatedly high readings
• Repeated readings below 70 mg/dL
• Symptoms of high or low blood sugar
• An abnormal A1C or fasting glucose result
• Unexplained changes in your usual numbers
• Pregnancy or possible gestational diabetes
• Difficulty staying within your individualized target range

For severe symptoms tied to very high or very low blood sugar — such as confusion, loss of consciousness, or trouble breathing — seek prompt medical care rather than trying to manage the situation on your own.

Frequently Asked Questions About Blood Sugar

What is blood sugar? Blood sugar is the glucose circulating in your bloodstream. It’s your body’s primary energy source, and it naturally changes throughout the day.

What is a normal blood sugar level? For someone without diabetes, a normal fasting blood sugar is below 100 mg/dL, and a normal level 2 hours after eating is below 140 mg/dL.

What is a normal blood sugar level in the morning? A normal fasting (morning) blood sugar for someone without diabetes is generally below 100 mg/dL. People with diabetes often have an individualized morning target, commonly around 80–130 mg/dL.

What blood sugar level indicates diabetes? Diagnostic thresholds include a fasting glucose of 126 mg/dL or higher, an A1C of 6.5% or higher, or a random glucose of 200 mg/dL or higher with symptoms — usually confirmed with a second test.

What is a normal blood sugar level after eating? Below 140 mg/dL at the 2-hour mark is considered normal for someone without diabetes.

What is considered low blood sugar? For people with diabetes, a reading below 70 mg/dL is generally considered low and may need treatment.

What is considered high blood sugar? This depends on context and your individual targets, but persistently elevated readings above your target range — or a fasting level of 126 mg/dL or higher — warrant a conversation with your doctor.

How is blood sugar tested? Common methods include a fasting blood glucose test, an A1C test, an oral glucose tolerance test, a random glucose test, and continuous glucose monitoring.

Is blood sugar the same as glucose? Yes. “Blood sugar” and “blood glucose” refer to the same thing: the glucose circulating in your blood.

What is the difference between blood sugar and A1C? A blood sugar reading is a snapshot at one moment. A1C reflects your average blood sugar over roughly the past 2–3 months.

How often should you check your blood sugar? This depends on your diabetes type, medications, and individual treatment plan. Your healthcare team can recommend an appropriate schedule.

Key Takeaways

• Blood sugar refers to the glucose circulating in your bloodstream.
• Blood glucose naturally changes throughout the day in response to food, activity, stress, and other factors.
• A1C, fasting glucose, oral glucose tolerance, and random glucose tests each provide different information.
• Diagnostic thresholds are different from the individualized targets used to manage existing diabetes.
• Repeated abnormal results deserve a conversation with a healthcare professional, not self-diagnosis.
• Nutrition, physical activity, medications, sleep, and stress can all influence blood glucose day to day.

If you’re still learning the basics, our overview of what is diabetes is a good next stop, along with our guide to metabolic syndrome if you’re managing multiple related risk factors.

This article is for general educational purposes and isn’t a substitute for personalized medical advice. Blood sugar targets vary by individual — talk with your doctor before interpreting your own results or changing your management plan.

Sources:

• Centers for Disease Control and Prevention (CDC) — Monitoring Your Blood Sugar
• American Diabetes Association — Diagnosing Diabetes and Learning About Prediabetes
• American Diabetes Association — Checking Your Blood Sugar
• National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, NIH) — Continuous Glucose Monitoring
• National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, NIH) — Low Blood Glucose (Hypoglycemia)
• National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, NIH) — Diet, Eating, and Physical Activity