Your fasting glucose test measures the amount of sugar in your blood after you have not eaten for at least 8 hours. A normal fasting glucose is below 100 mg/dL, and higher results signal prediabetes or diabetes.
| Category | Fasting glucose (mg/dL) |
|---|---|
| Normal | Below 100 |
| Prediabetes | 100 to 125 |
| Diabetes | 126 or higher |
Ranges follow ADA standards and vary slightly by lab; compare to your own report and confirm a diabetes-range result with repeat testing.
What fasting glucose measures
Fasting glucose reflects the concentration of sugar circulating in your blood after an overnight fast, typically 8 or more hours. It shows how well your body regulates blood sugar when you are not actively digesting food. Because it is a single point-in-time reading, it captures your baseline glucose control rather than day-to-day swings.
What high or low levels mean
A result of 100 to 125 mg/dL falls in the prediabetes range, and 126 mg/dL or higher on two separate tests indicates diabetes. Elevated fasting glucose usually points to insulin resistance, where your cells respond poorly to insulin, though acute stress, illness, or certain medications can also raise it temporarily. A low fasting glucose, generally below 70 mg/dL, is called hypoglycemia and may result from diabetes medications, prolonged fasting, excess alcohol, or less commonly a hormonal or metabolic condition. Follow up with your clinician if you see a value in the prediabetes or diabetes range, if you have symptoms like excessive thirst, frequent urination, or shakiness, or if a single reading looks unexpectedly high or low.
Fasting glucose FAQ
How long do I need to fast before the test?
You should not eat or drink anything except water for at least 8 hours before your blood is drawn. Most people schedule the test for the morning so the fast happens overnight. Coffee, juice, and gum can affect results.
Can one high reading mean I have diabetes?
Usually not. A single fasting glucose of 126 mg/dL or higher should be confirmed with a repeat test on a different day, unless you also have clear symptoms and a very high random glucose. One elevated value can reflect stress or illness.
What is the difference between fasting glucose and A1C?
Fasting glucose is a snapshot of your blood sugar at one moment, while hemoglobin A1C reflects your average glucose over roughly the past three months. Doctors often use both together to diagnose and monitor blood sugar problems. Read alongside Hemoglobin A1C, Triglycerides, and Non-HDL Cholesterol for a fuller picture of your metabolic health.
References
- American Diabetes Association, Diagnosis and Classification of Diabetes: Standards of Care in Diabetes.
- Cleveland Clinic, Prediabetes.
