Insulin

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.

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.

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

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.

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.