Fasting glucose

Many UK laboratories print 3.5 to 6.0 mmol/L for a sample taken after an overnight fast; some print an upper limit of 5.5. The range applies only if nothing but water was taken for at least eight hours.

This is the blood glucose measured after an overnight fast, usually eight to ten hours with nothing but water. Fasting removes the effect of the last meal, so the figure reflects how well the body keeps sugar steady between meals using stores in the liver and the hormone insulin.

Doctors request it when looking at how the body handles sugar, alongside or instead of HbA1c, and as part of health checks. It is also the first value of a glucose tolerance test.

• Diabetes and pre-diabetes
• Steroid medicines and some other medicines
• Stress, poor sleep and acute illness
• Being overweight and insulin resistance
• An overactive thyroid and some other hormone conditions

• Insulin and some diabetes tablets
• A very long fast, particularly with alcohol
• Hard exercise the evening before
• Rare tumours that make insulin, and some rare hormone conditions
• Severe liver disease

• Not having truly fasted is the commonest reason for a figure above the range
• A delay before the sample reaches the laboratory in the wrong tube makes it read low
• Stress, a poor night's sleep or an early morning coffee can nudge it up

Nothing but water for eight to ten hours before the sample, usually taken first thing in the morning. A single value above the range is normally repeated on another day before doctors act on it.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.