Glucose (Blood Sugar)
A blood test that measures the amount of glucose (sugar) in the blood, used to screen for and monitor diabetes and prediabetes.
Last medically reviewed: April 10, 2026 | Reviewed by LabResult.MD Editorial Team
What is Glucose?
Glucose is a type of sugar that serves as the body's main source of energy. It comes from the foods you eat, particularly carbohydrates, and is carried through the bloodstream to cells throughout the body. The hormone insulin, produced by the pancreas, helps cells absorb glucose from the blood.
A fasting blood glucose test measures the level of glucose in the blood after not eating for at least 8 hours. It is one of the most common tests for screening and monitoring diabetes and is often included in metabolic panels (BMP and CMP).
Why Is It Tested?
A healthcare provider may order a glucose test to:
- Screen for diabetes or prediabetes
- Monitor blood sugar control in people with known diabetes
- Evaluate symptoms such as excessive thirst, frequent urination, unexplained weight loss, or fatigue
- Assess blood sugar levels during pregnancy (gestational diabetes screening)
- Monitor the effects of diabetes medications or insulin therapy
- Evaluate as part of a routine health checkup, especially for adults over 45 or those with risk factors
Normal Reference Ranges
Reference ranges may vary by laboratory, testing method, age, and other factors. Always use the ranges provided on your lab report.
| Category | Fasting Glucose (mg/dL) | Fasting Glucose (mmol/L) |
|---|---|---|
| Normal | 70 – 99 mg/dL | 3.9 – 5.5 mmol/L |
| Prediabetes | 100 – 125 mg/dL | 5.6 – 6.9 mmol/L |
| Diabetes | 126 mg/dL or higher | 7.0 mmol/L or higher |
What Does a High Glucose Level Mean?
High blood glucose (hyperglycemia) means there is too much sugar in the blood. This is most commonly associated with diabetes, but can have other causes:
- Type 1 diabetes (the pancreas produces little or no insulin)
- Type 2 diabetes (the body becomes resistant to insulin)
- Prediabetes (blood sugar is elevated but not yet at diabetic levels)
- Gestational diabetes (diabetes that develops during pregnancy)
- Stress, illness, or infection (which can temporarily raise blood sugar)
- Certain medications (corticosteroids, diuretics, some antipsychotics)
- Cushing syndrome or other hormonal disorders
- Pancreatitis or pancreatic disease
A single high reading does not necessarily mean diabetes. A healthcare provider will typically confirm the diagnosis with repeat testing or additional tests like HbA1c or an oral glucose tolerance test.
What Does a Low Glucose Level Mean?
Low blood glucose (hypoglycemia) means blood sugar has dropped below normal levels. This can cause symptoms ranging from mild to severe. Common causes include:
- Taking too much insulin or diabetes medication
- Skipping meals or not eating enough carbohydrates
- Excessive physical activity without adequate food intake
- Excessive alcohol consumption
- Adrenal insufficiency or pituitary disorders
- Severe liver disease
- Rare insulin-producing tumors (insulinomas)
Symptoms of low blood sugar include shakiness, sweating, confusion, dizziness, irritability, and in severe cases, loss of consciousness. Immediate treatment involves consuming fast-acting carbohydrates (juice, glucose tablets).
Common Next Steps
Depending on the result, a healthcare provider may recommend:
- HbA1c (glycated hemoglobin) test to assess average blood sugar over the past 2–3 months
- Oral glucose tolerance test (OGTT) for a more detailed assessment
- Repeat fasting glucose test to confirm results
- Lifestyle changes including diet modification, exercise, and weight management
- Medications (metformin or insulin) if diagnosed with diabetes
- Regular monitoring and follow-up testing
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). Levels of 100–125 mg/dL indicate prediabetes, and 126 mg/dL or higher on two separate tests may indicate diabetes, according to the American Diabetes Association.
How often should blood glucose be tested?
The American Diabetes Association recommends screening for diabetes starting at age 35, or earlier if you have risk factors such as obesity, family history, or a history of gestational diabetes. People with normal results should be retested every 3 years.
What is the difference between glucose and HbA1c tests?
A fasting glucose test measures blood sugar at a single point in time, while an HbA1c test reflects average blood sugar over the past 2–3 months. Both tests are used to diagnose and monitor diabetes, and healthcare providers often use them together for a more complete picture.
Related Tests and Biomarkers
Sources
- American Diabetes Association. Standards of Medical Care in Diabetes (2024)
- MedlinePlus. Blood Glucose Test. U.S. National Library of Medicine
- Mayo Clinic. Blood sugar testing: Why, when and how
Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Reference ranges vary by laboratory. Always discuss your results with a qualified healthcare provider.