Fasting Glucose
Blood glucose after ≥ 8-hour fast – baseline marker for carbohydrate metabolism
Also known as
Definition
Fasting plasma glucose (after ≥ 8–12 hours without food) is the standard marker for evaluating carbohydrate metabolism. It reflects hepatic glucose production (gluconeogenesis, glycogenolysis) and baseline insulin action. It is globally standardized as a diagnostic criterion for diabetes mellitus and prediabetes. Together with fasting insulin, it allows calculation of the HOMA-IR as a measure of insulin resistance.
| Parameter | Value |
|---|---|
| Unit | mmol/l (mg/dl) |
| Reference Range | < 5.6 mmol/l (< 100 mg/dl): Normal | 5.6–6.9 mmol/l (100–125 mg/dl): Prediabetes (IFG) | ≥ 7.0 mmol/l (≥ 126 mg/dl): Diabetes mellitus (two measurements) |
Agree on your personal target range with your doctor.
Conversion: 1 mmol/l = 18.02 mg/dl. US labs use mg/dl (normal < 100 mg/dl, prediabetes 100–125 mg/dl, diabetes ≥ 126 mg/dl).
↓ What a low value means
Hypoglycemia (< 3.9 mmol/l / < 70 mg/dl) can indicate insulinoma, hypopituitarism, adrenal insufficiency, reactive hypoglycemia, or overdosage of insulin-stimulating medications. Symptoms: tremor, sweating, palpitations, confusion, loss of consciousness below 2.8 mmol/l.
↑ What a high value means
Elevated fasting glucose signals impaired carbohydrate metabolism. In the prediabetes range: reversible through lifestyle change. At ≥ 7.0 mmol/l: diabetes mellitus, high risk for micro- (retinopathy, nephropathy, neuropathy) and macroangiopathy (coronary heart disease, stroke).
✓ What can influence this value
Fasting glucose is influenced by fasting duration (measurement after at least 8 hours without food), body weight, physical activity, diet, sleep, stress, and conditions such as diabetes mellitus. Aerobic and strength training as well as weight loss are associated with lower fasting values. Glucose-lowering medications such as metformin change the value. The assessment is made by the treating doctor.
When to test
Reasons for testing include overweight, a family history of diabetes, a history of gestational diabetes, PCOS or high blood pressure. Your doctor decides how often it is measured. The measurement is taken in the morning after at least 8 hours without food, with no physical activity before the blood draw.
Frequently asked questions
How do fasting glucose and HbA1c relate to each other? +
Both are complementary: fasting glucose shows the current basal level (a daily snapshot), HbA1c reflects the 3-month average. HbA1c can be distorted by hemolytic conditions or anemia. The combination of both significantly increases sensitivity for early detection of prediabetes.
Why is my morning blood glucose sometimes higher than in the evening? +
This is the 'dawn phenomenon': cortisol, glucagon, and GH rise in the early morning hours (~4–8 am) and stimulate hepatic glucose production. This physiological rise provides energy for waking up. With insulin resistance or diabetes, the effect is more pronounced.
What is prediabetes? +
Prediabetes (fasting glucose 5.6–6.9 mmol/l or HbA1c 5.7–6.4%) is a reversible metabolic disorder. Studies show that 5–10% weight loss combined with 150 min/week of moderate activity reduces diabetes risk by 58% (DPP trial). Without intervention, ~25–70% of those with prediabetes develop type 2 diabetes within 3–5 years.
Fasting Glucose
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.