Insulin

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.