Fasting insulin

Most UK laboratories print the range in mU/L, commonly 2.6 to 24.9, which is 18 to 173 pmol/L. The range applies only to a fasting sample; the figure rises several-fold after food.

Insulin is the hormone made by the pancreas that moves glucose from the blood into the cells and keeps blood sugar steady. Measured after an overnight fast, it shows how much insulin the body is having to make to hold glucose at its resting level. A high fasting level with a normal glucose is a sign that the tissues are responding to insulin less readily than usual.

Doctors and private services request it, usually with a fasting glucose, when looking at insulin resistance, polycystic ovary syndrome, or unexplained low blood sugar. It is not a routine test.

• Any food or drink other than water in the hours before the test
• Being overweight, particularly around the waist, and insulin resistance
• Polycystic ovary syndrome
• Steroid medicines
• Rare tumours that make insulin
• Kidney disease, because insulin is cleared more slowly

• Type 1 diabetes and long-standing type 2 diabetes, where the pancreas makes less
• A prolonged fast or very low carbohydrate intake
• Regular exercise and weight loss
• Some diseases of the pancreas

• Not having truly fasted is the commonest reason for a high figure
• Insulin breaks down quickly if the sample is delayed, giving a falsely low result
• Methods differ widely between laboratories
• Insulin injections show up in some assays but not others

Nothing but water for eight to ten hours before the sample, taken in the morning. The sample must reach the laboratory promptly. Always read alongside the fasting glucose taken at the same time.

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