Glucose (random)

For a sample taken at any time of day without fasting, UK laboratories commonly print 3.5 to 7.8 mmol/L; a fasting sample has its own, lower range.

Glucose is the sugar that circulates in the blood and is the main fuel for the brain and muscles. It comes from the food that is eaten and from stores in the liver, and the hormone insulin keeps it within a narrow band. A random glucose is measured at any time of day, without fasting, so the figure depends partly on when the last meal was.

Doctors request it as part of routine blood tests, when looking at symptoms such as thirst, passing a lot of urine or tiredness, and to follow blood sugar in people who already have diabetes.

• A meal or sugary drink in the hours before the test
• Diabetes and pre-diabetes
• Stress, acute illness, infection and surgery
• Steroid medicines and some other medicines
• An overactive thyroid and some other hormone conditions
• Pregnancy, in some women

• Insulin and some diabetes tablets
• A long gap since eating, particularly with alcohol
• Very hard exercise
• Some rare hormone conditions and rare tumours that make insulin
• Severe liver disease

• The figure depends on how recently food was eaten
• If the sample sits for hours before it reaches the laboratory in the wrong tube, the red cells use up the glucose and the figure reads low
• Stress and lack of sleep push it up for a few hours

A random sample needs no preparation; a fasting sample means nothing but water for eight to ten hours. The laboratory uses a special tube that stops the sample using up its own glucose. A single value is read with the time of the last meal in mind.

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