Glucose (Blood Sugar)
A blood test that measures the amount of sugar (glucose) circulating in the bloodstream, used to screen for diabetes and monitor blood sugar control.
Last updated: April 10, 2026
Glucose is the body's main source of energy. A fasting blood glucose test measures how much sugar is in the blood after not eating for at least 8 hours. Normal fasting glucose falls between 70 and 99 mg/dL. Levels above this range may point to prediabetes or diabetes, while levels below 70 mg/dL may indicate hypoglycemia (low blood sugar). This test is one of the most commonly ordered lab tests in the world.
Why Is Glucose Tested?
Glucose testing is a cornerstone of preventive healthcare. The body breaks down carbohydrates from food into glucose, which enters the bloodstream and is used by cells for energy. The hormone insulin, produced by the pancreas, helps move glucose from the blood into cells. When this process does not work properly, blood sugar levels rise or fall outside the normal range.
A fasting glucose test is included in the Comprehensive Metabolic Panel (CMP) and the Basic Metabolic Panel (BMP). It is one of the primary screening tools for type 2 diabetes and prediabetes, conditions that affect more than 130 million Americans according to the CDC.
A healthcare provider may order this test as part of a routine checkup, when risk factors for diabetes are present (such as obesity, family history, or age over 35), or to monitor blood sugar in people already diagnosed with diabetes. It may also be ordered when symptoms such as increased thirst, frequent urination, unexplained weight loss, or fatigue are present.
Normal Reference Ranges
Reference ranges may vary by laboratory. Always use the ranges printed on your lab report. The values below apply to fasting blood glucose in adults.
| Category | Range | Unit |
|---|---|---|
| Adults (fasting) | 70 – 99 | mg/dL |
| Adults (fasting, international) | 3.9 – 5.5 | mmol/L |
| Children (fasting) | 70 – 100 | mg/dL |
To convert mg/dL to mmol/L: divide by 18. For example, 100 mg/dL = 5.6 mmol/L.
| Status | Fasting Glucose (mg/dL) |
|---|---|
| Normal | 70 – 99 |
| Borderline (Prediabetes) | 100 – 125 |
| High (Diabetes) | 126 or higher (on two separate tests) |
What Does a HIGH Glucose Level Mean?
A fasting glucose level of 100 mg/dL or higher is above the normal range and may indicate that the body is not processing blood sugar efficiently.
Possible causes of elevated glucose include:
- Prediabetes — fasting glucose between 100 and 125 mg/dL, indicating increased risk of developing type 2 diabetes
- Type 2 diabetes — fasting glucose of 126 mg/dL or higher on two separate occasions
- Type 1 diabetes — an autoimmune condition where the pancreas produces little or no insulin
- Gestational diabetes — high blood sugar that develops during pregnancy
- Stress response — physical illness, surgery, or emotional stress can temporarily raise blood sugar
- Medications — corticosteroids, certain diuretics, and some antipsychotic medications can increase glucose
- Cushing syndrome — a condition caused by prolonged exposure to high cortisol levels
- Pancreatitis — inflammation of the pancreas that can affect insulin production
Severity matters. A fasting glucose of 102 mg/dL is very different from 250 mg/dL. Mildly elevated results in the prediabetes range are often managed with lifestyle changes, while significantly elevated results may require medication and closer monitoring. A single high reading does not automatically mean diabetes — the diagnosis typically requires confirmation with a second test.
What Does a LOW Glucose Level Mean?
A fasting glucose level below 70 mg/dL is considered low and is called hypoglycemia. This means the body may not have enough sugar available for energy.
Possible causes of low glucose include:
- Diabetes medication — insulin or sulfonylureas can lower blood sugar too much, especially if a meal is skipped
- Prolonged fasting or not eating enough — going too long without food can deplete glucose stores
- Excessive alcohol consumption — alcohol can block glucose production in the liver
- Adrenal insufficiency — the adrenal glands do not produce enough cortisol, which helps regulate blood sugar
- Liver disease — the liver stores and releases glucose, so liver damage can impair this process
- Insulinoma — a rare tumor of the pancreas that produces excess insulin
- Intense physical activity — prolonged exercise can deplete blood sugar, especially without adequate food intake
Symptoms of low blood sugar may include shakiness, sweating, rapid heartbeat, confusion, dizziness, and irritability. Severe hypoglycemia (below 54 mg/dL) can cause seizures or loss of consciousness and requires immediate treatment. Mild low readings may simply reflect extended fasting before the blood draw.
Factors That Can Affect Your Results
Several factors beyond disease can influence a glucose test result. Understanding these can help put a result in context.
- Fasting status: Eating or drinking anything other than water within 8–12 hours before the test can raise results. This is the most common reason for a falsely elevated reading.
- Age: Fasting glucose tends to rise slightly with age. Older adults may have higher baseline levels that are still considered normal for their age group.
- Pregnancy: Hormonal changes during pregnancy can affect insulin sensitivity, leading to higher glucose levels. Gestational diabetes screening typically occurs between weeks 24 and 28.
- Medications: Corticosteroids, beta-blockers, thiazide diuretics, and certain antipsychotic drugs can raise blood sugar. Some diabetes medications can lower it too much.
- Stress and illness: Acute physical stress (infection, surgery, injury) or emotional stress triggers counter-regulatory hormones that raise blood sugar temporarily.
- Exercise: Vigorous exercise in the hours before the test can lower glucose levels. Regular physical activity improves insulin sensitivity over time.
- Time of day: Glucose levels are typically lowest in the early morning (the "dawn phenomenon" can cause a slight rise in some people). Testing is usually done in the morning after an overnight fast.
- Diet: A very high-carbohydrate meal the night before can influence fasting glucose the next morning. Long-term dietary patterns also affect baseline glucose levels.
Should I Be Concerned About My Glucose Level?
A single glucose reading that falls slightly outside the normal range is not necessarily a cause for alarm. Many temporary factors — what was eaten the night before, stress levels, medications, and even how long the fasting period lasted — can influence the result.
Context matters more than a single number. A healthcare provider will consider the glucose result alongside other information: A1C levels, symptoms, medical history, family history, body weight, and overall metabolic health. A borderline result often leads to a repeat test or an A1C test to get a more complete picture.
If a result does fall in the prediabetes range, this is actually encouraging news in many cases. Prediabetes is a stage where lifestyle changes — such as improving diet, increasing physical activity, and maintaining a healthy weight — have been shown to reduce the risk of progressing to type 2 diabetes by up to 58%, according to the Diabetes Prevention Program study.
Common Next Steps
Depending on the result, a healthcare provider may recommend one or more of the following:
- Hemoglobin A1C test — measures average blood sugar over the past 2–3 months (A1C vs. fasting glucose comparison)
- Oral glucose tolerance test (OGTT) — measures blood sugar response after drinking a glucose solution, often used during pregnancy screening
- Fasting insulin level — helps assess insulin resistance, especially when glucose is borderline
- Repeat fasting glucose — a second test to confirm an abnormal result
- Lifestyle modifications — dietary changes, regular exercise, and weight management are first-line interventions for prediabetes
- Medication — metformin or other glucose-lowering drugs may be recommended if lifestyle changes are not sufficient
- Additional CMP markers — review of kidney function (creatinine, BUN) and electrolytes since diabetes can affect multiple organ systems
Frequently Asked Questions
What is a normal fasting glucose level?
A normal fasting blood glucose level is between 70 and 99 mg/dL (3.9–5.5 mmol/L). This means the body is effectively managing blood sugar levels. The test requires fasting for at least 8 hours beforehand for accurate results.
What is the difference between glucose and A1C?
Fasting glucose is a snapshot of blood sugar at the time of the blood draw. A1C (hemoglobin A1C) measures the percentage of hemoglobin proteins coated with sugar, reflecting average blood sugar levels over the past 2–3 months. A1C does not require fasting. Both tests are used to diagnose and monitor diabetes, but they provide different information. Learn more in the A1C vs. fasting glucose comparison.
Does eating before the test affect results?
Yes. Eating or drinking anything other than water within 8–12 hours before a fasting glucose test can significantly raise blood sugar levels and produce an inaccurate result. If fasting instructions were not followed, the lab result may appear elevated even when glucose metabolism is normal. It is important to mention this to the ordering healthcare provider.
Can stress raise blood sugar?
Yes. Both physical stress (such as illness, infection, or surgery) and emotional stress trigger the release of hormones like cortisol and adrenaline. These hormones cause the liver to release stored glucose into the bloodstream, temporarily raising blood sugar. A single elevated reading during a stressful period may not reflect typical glucose control.
What is prediabetes?
Prediabetes is a condition where blood sugar is higher than normal but not yet high enough to be classified as type 2 diabetes. A fasting glucose of 100–125 mg/dL falls in the prediabetes range. An estimated 98 million American adults have prediabetes, and most do not know it. The good news is that with lifestyle changes such as improved diet, regular physical activity, and modest weight loss, prediabetes can often be reversed or its progression to diabetes significantly delayed.
How often should glucose be tested?
The American Diabetes Association recommends that screening for type 2 diabetes begin at age 35 for all adults, or earlier for those with risk factors such as being overweight, having a family history of diabetes, belonging to a high-risk ethnic group, or having a history of gestational diabetes. If results are normal, retesting every 3 years is generally recommended. Those with prediabetes may be tested annually.
Can glucose levels be too low?
Yes. A fasting glucose below 70 mg/dL is considered hypoglycemia (low blood sugar). Symptoms can include shakiness, sweating, rapid heartbeat, confusion, dizziness, and irritability. Severe hypoglycemia below 54 mg/dL can be dangerous and may cause seizures or loss of consciousness. Low blood sugar is most common in people taking diabetes medication, but it can also occur from prolonged fasting, excessive alcohol, or certain medical conditions.
Is glucose included in routine blood work?
Yes. Glucose is a standard component of both the Comprehensive Metabolic Panel (CMP) and the Basic Metabolic Panel (BMP). These panels are among the most commonly ordered lab tests and are frequently included in annual physicals and routine health screenings.
What is the difference between type 1 and type 2 diabetes?
Type 1 diabetes is an autoimmune condition where the immune system attacks insulin-producing cells in the pancreas, resulting in little or no insulin production. It usually develops in childhood or young adulthood. Type 2 diabetes is a metabolic condition where the body becomes resistant to insulin or does not produce enough. It is far more common (about 90–95% of diabetes cases) and is strongly linked to lifestyle factors. Both conditions cause elevated blood glucose, but they have different causes and treatments.
What units are used to measure glucose?
In the United States, glucose is measured in milligrams per deciliter (mg/dL). In most other countries, it is measured in millimoles per liter (mmol/L). To convert from mg/dL to mmol/L, divide by 18. For example, 100 mg/dL equals approximately 5.6 mmol/L. Lab reports will specify which unit is used.
Sources
- American Diabetes Association. Standards of Medical Care in Diabetes. Diabetes Care. 2024.
- Centers for Disease Control and Prevention. National Diabetes Statistics Report. 2022.
- Knowler WC, et al. Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med. 2002;346(6):393-403.
- MedlinePlus. Blood Glucose Test. U.S. National Library of Medicine.
- Mayo Clinic. Diabetes: Diagnosis and Treatment. 2024.
Related Tests and Biomarkers
Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Reference ranges vary by laboratory, testing method, and individual factors. Always discuss your results with a qualified healthcare provider.