Insulin

This test measures the amount of insulin in the blood, most often in the morning after fasting. Insulin is a pancreatic hormone that maintains blood glucose levels, and measuring it helps a doctor investigate low blood glucose or assess how much insulin the pancreas secretes. It can also provide additional information about insulin resistance, but is not a test that confirms it. It is always interpreted alongside glucose measured in the same sample.

Who the test is for

• people without diabetes whose blood glucose repeatedly falls too low;
• people whose doctor is assessing, together with C-peptide, how much insulin the pancreas still produces on its own;
• people who are overweight or have high blood lipids or blood pressure, when a doctor wants additional information about insulin resistance alongside other findings;
• women with polycystic ovary syndrome (PCOS) only when specifically indicated by a gynaecologist or endocrinologist, because insulin is not routinely measured in PCOS investigations.

What insulin is and what it indicates

Insulin is produced by pancreatic beta cells, found in groups of cells called the islets of Langerhans. The pancreas releases it when blood sugar (glucose) rises, for example after a meal. Insulin stimulates muscles and fat tissue to take up glucose from the blood, signals the liver to store it as a reserve (glycogen), and simultaneously inhibits new glucose production in the liver. When glucose falls, insulin secretion also decreases.

When cells respond less effectively to insulin, this is called insulin resistance. The pancreas must then release more insulin to keep glucose normal, so fasting insulin is often elevated while glucose is still normal. Insulin resistance often accompanies excess weight, particularly around the waist, polycystic ovary syndrome and metabolic syndrome, and increases the risk of type 2 diabetes over time.

In recurrent low blood glucose episodes (hypoglycaemia), a sample taken during the low glucose episode is important; this is often done in hospital. Insulin is not used to diagnose diabetes. Diagnosis is based on fasting glucose , HbA1c or a glucose tolerance test. The differences between insulin and C-peptide are described in The significance of measuring C-peptide and insulin.

HOMA-IR and OGTT with insulin

A doctor can calculate the HOMA-IR index from fasting glucose and insulin; it is not printed on the report: glucose (in mmol/L) multiplied by insulin (in mU/L) and divided by 22,5. The higher the value, the more likely insulin resistance is. However, there is no universal cut-off because the result depends on the insulin measurement method, age, body weight and the population in which the cut-off was determined. HOMA-IR is therefore a rough, supplementary estimate. It is not a standardised test for confirming insulin resistance in an individual or a diagnostic criterion for diabetes.

If a doctor wants to see how the pancreas responds to glucose, they may request a 4-point OGTT with insulin. After drinking a solution containing 75 g of glucose, glucose and insulin are measured in four blood samples. There are no universal cut-offs for insulin during the challenge either, so the result is assessed by a doctor.

Preparation and sample

• The sample is venous blood. Come in the morning after fasting for 8 to 12 hours unless your doctor requests otherwise. You may drink plain water, but no coffee, tea, juices or sweetened drinks.
• If your doctor wants to calculate HOMA-IR, glucose is measured alongside insulin because the index is calculated from both values measured in the same sample.
• Do not stop diabetes medicines yourself; agree with your doctor how to take them on the day of blood sampling. Also mention corticosteroids, because they can raise glucose and insulin.
• If you take biotin (e.g. supplements for hair and nails), tell staff at blood sampling.

How results are interpreted

The reference interval depends on the method and is printed on the report. Insulin is always assessed alongside glucose: the same insulin value may be appropriate with higher glucose but too high when glucose is low.

• High fasting insulin with normal or mildly elevated glucose may suggest insulin resistance, but the result alone does not confirm it. A doctor assesses it alongside other findings.
• High insulin with low glucose requires further medical investigation. Causes may include medicines that stimulate insulin secretion, externally administered insulin or, rarely, an insulin-secreting pancreatic tumour.
• Low insulin with elevated glucose means the pancreas secretes too little insulin, which is typical of type 1 diabetes and long-standing type 2 diabetes.
• Low insulin with low or normal glucose is usually expected because a healthy pancreas secretes little insulin in these circumstances.

People treated with insulin also have injected insulin in their blood, so insulin measurement does not show how much the pancreas produces; C-peptide is measured instead. One result is not a diagnosis: a doctor interprets it alongside glucose, HbA1c, body weight, waist circumference, blood pressure and lipids.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed on the Contact page). Prices for all tests and panels are available in the service price list.

Frequently asked questions

Is there a test for insulin resistance?

There is no single universal test. Fasting insulin and glucose, used to calculate HOMA-IR, and an OGTT with insulin can provide additional information. They are not universal tests for confirming insulin resistance and are not routinely recommended, for example in PCOS investigations. Results are assessed alongside body weight, waist circumference, blood pressure and lipids, rather than a single number.

What does high fasting insulin mean?

It often means the body needs more insulin to keep glucose normal, which may suggest insulin resistance. The result alone does not confirm it. Insulin resistance is not diabetes but increases the risk of it. Regular physical activity, dietary changes and losing excess weight reduce risk, and a doctor will assess whether additional tests are needed.

Do I need to fast for an insulin test?

Yes, unless your doctor requests otherwise. Insulin rises soon after eating, so a value measured after a meal cannot be compared with the reference interval or used for HOMA-IR.

Is it useful to measure insulin if I am treated with insulin?

Usually not, because the result also includes injected insulin. When a doctor wants to know how much insulin the pancreas still produces on its own, they request C-peptide.