Glucose is the main fuel for cells. Its blood level is held tightly within a narrow corridor: insulin lowers it, glucagon and cortisol raise it.
A fasting test shows the baseline level — how the body holds sugar at rest, without a food load. It is the first and most accessible marker of carbohydrate-metabolism disorders.
Reference ranges
Venous plasma, fasting (8–12 hours without food):
| Normal | 3.9 – 6.1 mmol/L |
| Metabolic syndrome criterion | 5.6 mmol/L and above |
| Impaired fasting glucose (prediabetes) | 6.1 – 6.9 mmol/L |
| Diabetes threshold | 7.0 mmol/L and above |
| Hypoglycemia | below 3.9 mmol/L |
What an elevated level means
Elevated fasting glucose means insulin is no longer keeping up. This is usually a consequence of insulin resistance, which develops over years before the numbers show up on a test.
The 6.1–6.9 mmol/L range is prediabetes: a reversible state where a change of lifestyle works better than any medication. From 7.0 it is the diabetic threshold (needs confirmation).
Chronically high sugar damages vessels, kidneys, nerves and the retina and accelerates aging through protein glycation.
What a low level means
Glucose below 3.9 mmol/L is hypoglycemia. In healthy people not being treated for diabetes it is rare.
Causes: an overdose of glucose-lowering drugs or insulin, prolonged fasting, alcohol, severe liver and kidney disease, adrenal insufficiency, rare tumors (insulinoma).
Symptoms: trembling, sweating, hunger, palpitations, confusion. In the moment this state is more dangerous than moderate hyperglycemia and requires immediate action.
How the test is done
Blood is drawn fasting: 8–12 hours without food; water is allowed. Juice, tea and coffee are already food, even without sugar.
Test in the morning: glucose has a daily rhythm. Acute illness, stress, a sleepless night and intense exercise the day before can noticeably raise the result — under such conditions the test is best postponed.
What affects the value
- Diet — an excess of refined carbohydrates and an overall calorie excess.
- Visceral fat and insulin resistance — the main mechanism behind rising sugar.
- Stress and cortisol — raise glucose directly.
- Lack of sleep — even one sleepless night lowers insulin sensitivity.
- Physical activity — muscles take up glucose without insulin.
- Medications — glucocorticoids, thiazide diuretics, some antipsychotics.
Calculate using this marker
Calculators where Fasting glucose is used directly:
Frequently asked questions
Is glucose of 5.8 normal?
Formally it is still normal (up to 6.1), but it is already above the 5.6 threshold that counts as a criterion of metabolic syndrome. This is "high-normal" — a reason to check HbA1c, waist and lipids, not a reason to relax.
How does fasting glucose differ from HbA1c?
Glucose is a snapshot of "here and now" and is easily distorted by stress, lack of sleep or breaking the fast. HbA1c shows the average sugar over 2–3 months and does not depend on one-off swings. They are looked at together.
Can I drink water before the test?
Yes, plain water is allowed and even recommended. But juice, tea, coffee (even unsweetened) and chewing gum break the "fasting" condition and can distort the result.