Glucose — Blood Sugar (Fasting & Random)

What is blood glucose?

Glucose is the body's primary energy source, derived from carbohydrates in the food we eat. After digestion, glucose enters the bloodstream, triggering the pancreas to release insulin — a hormone that enables cells to take up and use glucose for energy. Any surplus is stored in the liver and muscles as glycogen, or converted to fat.

A fasting blood glucose test measures the concentration of glucose in the blood after at least 8 hours without eating or drinking anything other than water. This removes the variability caused by recent meals, giving a clean baseline reading of how well the body regulates blood sugar in the resting state. The result is the primary diagnostic criterion for diabetes mellitus and pre-diabetes (impaired fasting glucose).

A random (non-fasting) glucose test can be taken at any time and is used in emergency or screening contexts. It is less precise for diagnosing pre-diabetes but a very high random result (≥11.1 mmol/L with symptoms) is sufficient to diagnose diabetes. For the fullest picture, glucose is best combined with HbA1c, which reflects average blood sugar over the preceding 8–12 weeks.

Normal blood glucose range

Glucose is measured in mmol/L (millimoles per litre). The WHO and NICE diagnostic thresholds are:

• Fasting glucose 3.9–5.5 mmol/L — Normal
• Fasting glucose 5.6–6.9 mmol/L — Pre-diabetes (Impaired Fasting Glucose)
• Fasting glucose ≥7.0 mmol/L — Diabetes mellitus (requires confirmation on a second test if asymptomatic)
• Random glucose ≥11.1 mmol/L — Diabetes mellitus (with typical symptoms)
• Fasting glucose <3.9 mmol/L — Hypoglycaemia (low blood sugar)

What a high glucose level may indicate

A raised fasting glucose level above 5.5 mmol/L warrants clinical attention. Possible causes include:

• Pre-diabetes (IFG) — fasting 5.6–6.9 mmol/L; the pancreas is struggling to keep pace; lifestyle changes at this stage can prevent progression to diabetes
• Type 2 diabetes mellitus — the most common cause of persistently raised fasting glucose; insulin resistance is typically the underlying mechanism
• Type 1 diabetes mellitus — autoimmune destruction of insulin-producing beta cells; usually presents acutely with marked hyperglycaemia
• Steroid medication — corticosteroids (e.g. prednisolone, dexamethasone) significantly raise blood glucose and can cause steroid-induced diabetes
• Acute illness or physiological stress — infection, surgery, or trauma temporarily raises glucose through counter-regulatory hormones such as adrenaline and cortisol

What a low glucose level may indicate

Hypoglycaemia (blood glucose below 3.9 mmol/L) means the blood sugar has fallen too low for cells to function normally. Causes include:

• Insulin or diabetes medication — the most common cause; too high a dose relative to food intake or activity level
• Prolonged fasting or starvation — glycogen stores are depleted and glucose production falls
• Excessive alcohol — inhibits hepatic glucose production (gluconeogenesis), particularly when combined with fasting
• Insulinoma — a rare insulin-secreting tumour of the pancreas; causes recurrent spontaneous hypoglycaemia
• Addison's disease — low cortisol impairs counter-regulatory glucose response

Get your glucose tested at The Vesey

Fasting glucose is included in the following panels at The Vesey Private Hospital, Sutton Coldfield:

• Diabetes Status Panel — fasting glucose, HbA1c, and insulin resistance assessment
• Lipids & HbA1c Panel — £149 (includes fasting glucose + HbA1c + full lipid profile)
• Lifestyle Screen — comprehensive multi-system panel including glucose and HbA1c

Fasting required: Please fast for at least 8 hours before your appointment (water is fine). Morning appointments are recommended. We offer early slots from 8am including weekends.

Frequently asked questions

Further reading: Glucose — Lab Tests Online UK · Independent patient information from the British Society for Clinical Biochemistry.