42 Glucose

Blood glucose: normal fasting values, prediabetes and diabetes criteria (ADA), hypoglycaemia and emergency signs.

Blood glucose is the concentration of sugar in the blood and the main fuel of the brain. It is kept within a narrow range by hormones: insulin from the pancreas lowers it after meals, while glucagon, adrenaline, cortisol and growth hormone raise it when needed. Persistently high glucose is the defining feature of diabetes mellitus, which damages blood vessels, eyes, kidneys and nerves. Too low glucose (hypoglycaemia) quickly impairs brain function.

MetabolismDiabetesHypoglycaemia

Normal fasting3.9–5.5 mmol/L (70–99 mg/dL)

Prediabetes (fasting)5.6–6.9 mmol/L (100–125 mg/dL)

Diabetes (fasting)≥ 7.0 mmol/L (≥ 126 mg/dL)

Hypoglycaemia< 3.9 mmol/L (< 70 mg/dL)

Figure 1. Fasting plasma glucose categories according to the American Diabetes Association. Fasting means no calorie intake for at least 8 hours.

Diagnosing diabetes

According to the American Diabetes Association, diabetes is diagnosed when fasting plasma glucose is ≥ 7.0 mmol/L (126 mg/dL), or HbA1c is ≥ 6.5 %, or glucose 2 hours after a 75 g glucose drink is ≥ 11.1 mmol/L (200 mg/dL). Without obvious symptoms, the result must be confirmed by a second abnormal test. A random glucose ≥ 11.1 mmol/L with classic symptoms is diagnostic on its own. Prediabetes means a fasting glucose of 5.6–6.9 mmol/L or an HbA1c of 5.7–6.4 %. Lifestyle changes at this stage substantially reduce progression to diabetes.

Causes of high glucose

Type 2 diabetes (insulin resistance with insufficient insulin), type 1 diabetes (autoimmune loss of insulin production), gestational diabetes
Stress hyperglycaemia during acute illness, infection, surgery or heart attack
Medicines: corticosteroids, some antipsychotics, thiazides, tacrolimus
Pancreatic disease, Cushing syndrome, acromegaly, pheochromocytoma
A non-fasting sample (after eating, glucose is normally higher, usually below 7.8 mmol/L after 2 hours)

Low glucose — hypoglycaemia

Symptoms include shaking, sweating, hunger, palpitations, anxiety, then confusion, unusual behaviour, seizures and loss of consciousness. Most episodes occur in people treated with insulin or sulfonylureas. Other causes include alcohol, liver failure, kidney failure, adrenal insufficiency, rare insulin-producing tumours, and low glucose after gastric surgery. Glucose below 3.0 mmol/L (54 mg/dL) is clinically significant hypoglycaemia.

Seek emergency care

for confusion, drowsiness or loss of consciousness with low glucose, or for very high glucose with vomiting, abdominal pain, deep rapid breathing, severe thirst or drowsiness (possible diabetic ketoacidosis or hyperosmolar state). If someone with suspected hypoglycaemia is conscious and can swallow, give fast-acting sugar (juice, glucose tablets).

Planned evaluation

A fasting glucose of 5.6 mmol/L or more should be followed up with HbA1c and assessment of weight, blood pressure and lipids. Regular activity, weight loss and a diet low in refined sugar reduce the risk of diabetes.

References

American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2025. Diabetes Care. 2025;48(Suppl 1):S27–S49. doi:10.2337/dc25-S002
American Diabetes Association Professional Practice Committee. 6. Glycemic goals and hypoglycemia: Standards of Care in Diabetes—2025. Diabetes Care. 2025;48(Suppl 1):S128–S145. PMID: 39651981

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 13 May 2024

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