Also Known As
Fasting Insulin
Formal Name
Insulin, serum
This article was last reviewed on
This article waslast modified on 27 August 2021.
At a Glance
Why Get Tested?

To help determine the cause of low blood glucose (hypoglycaemia), diagnose an insulinoma (insulin-producing tumour), and to help evaluate insulin production.

When To Get Tested?

If you have hypoglycaemia, if you have symptoms suggesting insulin is being inappropriately produced by your body, and rarely if you have diabetes and your doctor wants to monitor your insulin production

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

You may be asked to fast for 8 hours before the blood sample is collected, but occasionally a healthcare professional may do the test in very specific circumstances, for example, a glucose tolerance test. In some cases, a healthcare professional may request that you fast longer.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Insulin is a protein hormone that is produced and stored in the beta cells of the pancreas. It is initially made as a larger molecule, preproinsulin, that is cleaved twice to form the smaller insulin hormone prior to release and inactive fragments proinsulin and C-peptide.

When blood glucose levels rise after a meal, insulin causes glucose to be taken up by the body's cells, especially muscle, liver and fat cells, where is it is used for energy production and storage. Insulin is required to regulate blood glucose concentrations and plays a role in controlling the levels of carbohydrates and fats stored in the body.

Humans need insulin on a daily basis to survive. Without insulin, glucose cannot reach most of the body's cells. Without glucose, the cells starve, and blood glucose levels rise to dangerous levels. Eventually, very high glucose concentrations lead to a life-threatening condition called a diabetic coma.

Insulin and glucose levels must be in balance. Too much insulin in the blood, for the blood glucose level, is known as 'hyperinsulinaemia'. This is seen with insulinomas (insulin-producing tumours usually found within the pancreas) or, more commonly, with an excess amount of injected insulin or other diabetic drugs stimulating insulin release. Hyperinsulinaemia causes hypoglycaemia (low blood glucose levels), which can lead to sweating, rapid heart beat, hunger, confusion, visual problems, and seizures. Since the brain is totally dependent on blood glucose as an energy source, lack of glucose due to hyperinsulinaemia can lead fairly quickly to confusion, coma, brain damage and/or death.

Insulin also signals to the body that there is ‘fuel’ available. When no ‘fuel’ is available (in the form of glucose) and there is no insulin the body uses fats to make ketones which are the only other fuel the brain can use. In hyperinsulinaemia the body is unable to respond to the low glucose by making ketones and so the brain suffers. Therefore if ketones can be detected in the blood, in someone with a low glucose, an insulin test is no longer needed as it must be absent.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. It is almost always necessary to take a sample for glucose at the same time to help interpretation. The blood sample for glucose measurement will probably be collected into a different tube.

Is any test preparation needed to ensure the quality of the sample?

Typically, a person will be asked to fast for 8 hours before blood is collected, but occasionally a healthcare professional may do insulin testing when food is present such as during a glucose tolerance or mixed meal test or at the time of an episode of low blood sugar (hypoglycaemia). In some cases, the healthcare professional may request that a person fast longer than 8 hours (e.g. 72 hours) and may take steps to trigger a hypoglycaemic episode.

Accordion Title
Common Questions
  • How is it used?

    Insulin and C-peptide are produced by the body at the same rate. Both may be requested to evaluate how much insulin in the blood is due to endogenous production (what your body is making) and how much is from exogenous (produced outside the body, e.g. injected) sources. Insulin tests will reflect the total level, while C-peptide will reflect only the insulin made by the body.

    The primary use of insulin measurement, along with glucose and C-peptide levels, is to investigate the cause of hypoglycaemia (low blood glucose). One cause of hypoglycaemia is an insulin secreting tumour (insulinoma) and so insulin and C-peptide will both be high. Another cause of hypoglycaemia is injection of insulin, which will cause hyperinsulinaemia during hypoglycaemia with a suppressed C-peptide. Other diabetes medications e.g. sulphonylureas, cause the pancreas to release insulin, therefore a high C-peptide level does not always mean a patient has an insulinoma. Sulphonylureas can be tested for which may help to tell the difference between medication and insulinoma as a causing for low blood sugars.

    Bariatric surgery may also cause hypoglycaemia. The extensive abdominal surgery can disrupt the body’s ability to handle food properly. This can result in rapid changes in blood sugar including hypoglycaemia, part of the spectrum of ‘dumping syndrome’. Rarely after bariatric surgery the pancreas grows and produces too much insulin (nesidioblastosis) so insulin measurement might be done.

    Insulin and C-peptide concentrations also may be used to monitor the amount of insulin produced by the body (called 'endogenous' insulin), to check if the body is not responding to insulin properly (called 'insulin resistance'). The use of insulin measurement here is controversial. The problems associated with insulin resistance, namely high blood pressure, high cholesterol and hyperglycaemia, are easier to measure directly and the concentration of insulin is not directly related to these complications. Therefore most guidelines do not recommend the measurement of insulin to routinely assess for insulin resistance but instead recommend the use of body mass index, waist circumference, blood pressure, lipid profile and diabetes assessments.

    Insulin concentrations are sometimes used in conjunction with the glucose tolerance test (GTT). Insulin concentrations may also be included in other ‘dynamic’ tests such as prolonged fasts, where an individual is asked not to eat for 72 hours or until a hypoglycaemic episode occurs, and mixed meal tests.

  • When is it requested?

    Insulin concentrations are most frequently requested following an abnormal glucose test and/or when a patient has short- or long-term symptoms of hypoglycaemia, such as sweating, palpitations, hunger, confusion, visual problems, and seizures (although these can be caused by other conditions). 

    Your doctor also may request both insulin and C-peptide tests to check that an insulinoma has been successfully removed. If you are one of the few people who have received a pancreas cell transplant to restore your ability to produce insulin, your insulin level may be monitored to determine whether or not this procedure is successful over time. Measurement of blood glucose levels, however, is the mainstay of monitoring after a pancreas transplant.

    The type of diabetes mellitus that a person has is usually very clear after speaking to the person, examining them and looking at the routine tests. In rare cases, e.g. an individual appears to have type 2 diabetes but presented with ‘ketoacidosis’ which normally only affects those with type 1 diabetes, then insulin, but more likely C-peptide levels may be helpful.

  • What does the test result mean?

    Insulin results must be evaluated in context. If fasting insulin and glucose levels are normal, most likely the body's glucose regulation system is functioning normally. If insulin is raised and glucose is normal and/or moderately raised, then there may be some insulin resistance. If the insulin is low and the glucose is high, then most likely there is insufficient insulin being produced by the body. If insulin levels are normal or raised and glucose levels are low, then the patient is hypoglycaemic due to excess insulin.

    Raised insulin concentrations may be seen with:

    1. Acromegaly
    2. Cushing's syndrome
    3. Drugs such as corticosteroids, levodopa, oral contraceptives
    4. Fructose or galactose intolerance
    5. Insulinomas
    6. Obesity
    7. Insulin resistance, such as appears in early type 2 diabetes
    8. Analytical interference, such as from exogenous insulin administration or insulin autoantibodies

    Decreased insulin concentrations are seen with:

    1. Diabetes
    2. Hypopituitarism

  • Is there anything else I should know?

    Insulin for injection (exogenous insulin) used to come from animals but now most is synthetic and made to match identically the insulin produced by human beta cells (endogenous insulin). The different insulin preparations however have different properties such as speed of onset or duration allowing mixtures and/or different types of insulin to be taken by those with diabetes throughout the day to mimic the body’s normal response to food and fasting. Due to the similarities between exogenous and endogenous insulin some assays are unable to tell the difference which can complicate the interpretation of results.

    Different insulin tests are also not identical meaning there are differences in how well they can detect the different kinds of insulin and if one blood sample was sent to different laboratories each would give a different result. If your insulin result is not what your doctor was expecting, he or she may want to talk to the laboratory that performed your test to discuss the interpretation. If you are going to have several insulin tests done, they should be performed by the same laboratory to ensure consistency.

    If you have developed anti-insulin antibodies, they can also interfere with your test results and need to be removed before an accurate insulin level can be measured. Any breakdown of the red blood cells in the sample, or delay in the separation of the liquid in the sample from the red cells, will cause the insulin to be broken down and therefore a repeat sample may be required.

  • Can I do an insulin test at home?

    No. Although glucose levels can be monitored at home, insulin tests require specialised equipment and samples are likely to be sent to a specialised laboratory for analysis and interpretation.

  • Why does insulin have to be injected?

    Insulin must be injected or given via an insulin pump. It cannot be given orally (by mouth) because it breaks down in the stomach and will never reach the blood stream where it needs to be to work.

  • How is an insulinoma treated?

    Insulinomas are insulin-producing tumours but are most often not malignant. They are usually treated by being located and removed. Once removed, generally they do not return.

  • What is Insulin Resistance?

    Insulin resistance is a decreased ability of insulin to stimulate transport of glucose into the body’s cells where it is needed for energy production. The pancreas tries to compensate for the cells' lack of glucose by producing more insulin. This results in elevated levels of insulin and C-peptide in the blood along with normal or elevated glucose levels. Glucose, insulin, and C-peptide levels may help your doctor diagnose this condition.

    Insulin resistance is a warning signal that the body is having problems processing glucose, and is at risk of developing diabetes. Patients with early or moderate insulin resistance often don’t have any symptoms, but if their condition is ignored, it puts them at a much greater risk of developing type 2 diabetes, high blood pressure, hyperlipidaemia, and/or heart disease several years down the road. (These four conditions make up what is sometimes called the Metabolic Syndrome).

    Risk factors for insulin resistance include:

    • Obesity, especially abdominal obesity (i.e. round the waist)
    • Family history of diabetes
    • Gestational diabetes (diabetes when pregnant)
    • Polycystic ovary syndrome

    Treatment of insulin resistance involves changes in diet and lifestyle. Diabetes UK recommends losing excess weight and taking regular amounts of exercise to lower blood insulin levels and increase the body’s sensitivity to insulin.