C-peptide is a protein that the body produces at the same time as it produces insulin, the hormone that helps the body use glucose and control its levels in the bloodstream. C-peptide is responsible for linking A and B insulin chains and facilitating the efficient processing of insulin in the cell. This protein is usually found in amounts equivalent to the insulin produced, making it a reliable marker of how much insulin the body is creating. 

The C-peptide test is most often used to determine the cause of low blood sugar and monitor insulin production. While it is almost never ordered to diagnose diabetes, it may be used in people who have recently had a diabetes diagnosis, to determine how much insulin the pancreas is capable of producing. In people with Type 1 diabetes, a condition that causes the body to stop producing insulin, C-peptide levels may be quite low. In people with Type 2 diabetes, where the body becomes resistant to its own insulin, C-peptide levels may be very high. 

C-peptide testing can also be used to determine the effectiveness of insulin treatment, since the body can develop antibodies to insulin that interfere with testing for the hormone itself. This test also helps evaluate people who have metabolic syndrome, which can eventually lead to more serious conditions. Multiple C-peptide tests should always be done by the same laboratory, using the same method. Patients should also be aware that insulin deteriorates faster than C-peptide in the bloodstream, so normal levels are about five times the serum insulin level.