A test that measures the amount of insulin peptide in a blood or urine sample. Insulin peptide is an inactive byproduct of insulin production by the pancreas. This test can help your doctor determine if you have type 1 diabetes, in which the immune system destroys cells in the pancreas that make insulin; or type 2, in which your body doesn’t use insulin as well as it should. The test can also help your doctor decide how much insulin you need to take.
Insulin is a hormone that moves sugar (or glucose) from the blood into cells to use for energy. Insulin is produced by specialized cells in the pancreas called beta cells. The process of making insulin involves first creating proinsulin, which then gets split apart to form one molecule of C-peptide and one molecule of insulin.
Within the cell, insulin binds to the insulin receptor and triggers a series of protein activation cascades that include translocation of the GLUT-4 transporter to the membrane and influx of glucose, glycogen synthesis, and metabolism of fatty acids and triglycerides. Insulin also has antigen-binding properties and can induce the proliferation of specific cells, resulting in formation of new beta cells.
A health care provider orders a C-peptide test to help assess how much insulin a person with diabetes needs and whether the person should start taking insulin injections or change to another type of diabetes medicine. A blood draw for this test is a relatively quick and low-risk procedure. Some people feel moderate pain or stinging when the needle is inserted, and there may be a slight bruise at the site where the blood sample was drawn.