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Fasting Glucose – Definition, Normal Values and Diagnosis

Fasting glucose is the blood sugar level measured after at least 8 hours without food intake. It is a key test for diagnosing diabetes mellitus and prediabetes.

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Things worth knowing about "Fasting Glucose"

Fasting glucose is the blood sugar level measured after at least 8 hours without food intake. It is a key test for diagnosing diabetes mellitus and prediabetes.

What Is Fasting Glucose?

Fasting glucose (also called fasting blood sugar or fasting plasma glucose) refers to the concentration of glucose in the blood after a fasting period of at least 8 hours, typically overnight. Because no food has been consumed during this time, the value reflects the body's baseline glucose regulation, independent of recent dietary intake. Fasting glucose is one of the most important diagnostic parameters in diabetology and internal medicine.

Normal Values and Thresholds

Fasting glucose results are interpreted according to internationally recognized reference ranges:

Normal range: below 100 mg/dL (5.6 mmol/L)
Impaired fasting glucose (prediabetes): 100–125 mg/dL (5.6–6.9 mmol/L)
Diabetes mellitus: 126 mg/dL (7.0 mmol/L) or higher, confirmed on at least two separate occasions

These thresholds are based on guidelines from the World Health Organization (WHO) and the American Diabetes Association (ADA).

Clinical Significance

Fasting glucose is a valuable tool for early detection of metabolic disorders. Persistently elevated fasting glucose indicates impaired insulin action or production and may point to the following conditions:

Prediabetes: A precursor to type 2 diabetes, where lifestyle changes can be highly effective
Type 2 diabetes mellitus: The most common form of diabetes, frequently associated with insulin resistance
Type 1 diabetes mellitus: An autoimmune condition characterized by absolute insulin deficiency
Gestational diabetes: Elevated blood glucose levels occurring during pregnancy
Secondary diabetes: Caused by pancreatic disease or certain medications

How the Test Is Performed

To ensure an accurate fasting glucose measurement, the following conditions must be met:

At least 8 hours of fasting before the blood draw
Plain water (without sugar or caffeine) may be consumed
Blood is typically collected in the morning from a vein (venous plasma) or via a fingerstick (capillary whole blood)
Intense physical activity or significant stress before the test should be avoided, as both can affect blood glucose levels

Venous plasma glucose is considered the gold standard for diagnosis. Values obtained from capillary blood or home glucose meters may differ slightly.

Diagnosis and Follow-Up Testing

If the fasting glucose value falls in a borderline or elevated range, further diagnostic steps may be indicated:

Oral glucose tolerance test (OGTT): Measures blood glucose before and after ingesting a glucose solution to assess how the body processes sugar
HbA1c (glycated hemoglobin): Reflects average blood glucose over the past 2–3 months and complements the fasting glucose result
Repeat measurement: A single elevated fasting glucose value must be confirmed on a separate day before a diagnosis of diabetes is made

Treatment and Prevention

When impaired fasting glucose or diabetes mellitus is identified, several management strategies are available:

Lifestyle Modifications

A healthy, balanced diet with reduced intake of sugar and refined carbohydrates
Regular physical activity (at least 150 minutes of moderate exercise per week)
Weight loss in individuals with overweight or obesity
Avoiding smoking and excessive alcohol consumption

Medical Treatment

In cases of established diabetes mellitus, blood glucose-lowering medications such as metformin, SGLT-2 inhibitors, or GLP-1 receptor agonists may be prescribed, depending on the type and severity of the condition. Insulin therapy may also be required. Treatment is always individualized and determined by a physician.

References

World Health Organization (WHO): Definition and Diagnosis of Diabetes Mellitus and Intermediate Hyperglycaemia. WHO Press, Geneva, 2006.
American Diabetes Association (ADA): Standards of Medical Care in Diabetes – 2024. Diabetes Care, 47 (Suppl. 1), 2024.
Rydén L. et al. – ESC Guidelines on diabetes, pre-diabetes, and cardiovascular diseases. European Heart Journal, 2023.

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