Fasting Blood Glucose: Normal Range, High & Low Causes
What Fasting glucose measures
A fasting glucose test measures the sugar in your blood after you have not eaten for at least eight hours. It is a common screen for prediabetes and diabetes.
Fasting glucose reference ranges
| Category | Conventional units | SI units |
|---|---|---|
| Low (hypoglycemia) | Below 70 mg/dL | Below 3.9 mmol/L |
| Normal | 70 – 99 mg/dL | 3.9 – 5.5 mmol/L |
| Prediabetes (impaired fasting glucose) | 100 – 125 mg/dL | 5.6 – 6.9 mmol/L |
| Diabetes range | 126 mg/dL or higher | 7.0 mmol/L or higher |
A diagnosis normally needs a repeat test or a second kind of test, such as HbA1c, on a different day.
Need to convert units? Use the mg/dL ↔ mmol/L converter.
Common causes of a high result
- Diabetes or prediabetes
- Not fasting long enough before the test
- Acute illness or physical stress
- Steroid medicines and some other drugs
- Pancreatic conditions or hormone disorders
Common causes of a low result
- Diabetes medicines such as insulin or sulfonylureas
- Skipping meals or prolonged fasting
- Heavy alcohol use
- Rare hormone or insulin-producing tumours
How to prepare for the test
Do not eat or drink anything except water for at least 8 hours before the test unless your clinician tells you otherwise.
How often to retest
If the result is in the prediabetes range, clinicians often recheck it at least yearly.
Track Fasting glucose over time
Upload your reports and Labgist plots every Fasting glucose result on one timeline, converts units and shows what changed.
Get the appFrequently asked questions
What is a normal fasting blood sugar?
70–99 mg/dL (3.9–5.5 mmol/L) after at least eight hours without food.
Does coffee break a fast for a glucose test?
Plain water is the safest choice. Even black coffee can affect some people, so follow the instructions from your lab or clinician.
Is 105 mg/dL fasting glucose bad?
105 mg/dL (5.8 mmol/L) is in the prediabetes range. It is a prompt to talk with your clinician about lifestyle changes and follow-up testing, not a diabetes diagnosis on its own.
Related tests
Sources
This page is general education and was last updated 2026-10-30. It is not a substitute for medical advice. See our editorial approach.