C-Peptide

C-peptide is a small protein that is released into your bloodstream at the same time and in the same amount as the insulin your pancreas makes. This test measures the C-peptide in your blood to show how much of your own insulin your body is producing. Injected insulin does not contain C-peptide, so the result reflects only your natural insulin production.

Your care team may order this test to evaluate how well the insulin-producing beta cells in your pancreas are working. It can be collected while fasting or after a stimulation procedure, depending on the question being asked.

C-peptide helps distinguish reduced insulin production from insulin resistance. It is often used when figuring out the type of diabetes, monitoring remaining beta cell function over time, and guiding decisions about treatments that support or replace insulin. It can also be useful after pancreas surgery or in long-standing diabetes to assess remaining function.

The test is also used when investigating low blood sugar to tell whether insulin is coming from your body or from an outside source. Doctors may use it when evaluating possible insulin-producing tumors, medication effects, or other endocrine conditions that affect insulin secretion.

A lower result usually suggests your pancreas is making little insulin, which can be seen in conditions that damage beta cells or after pancreatic surgery. A higher result may suggest your body is making a lot of insulin, as can occur with insulin resistance or with certain tumors or medications. Kidney problems can increase levels because C-peptide is cleared through the kidneys, and eating before the test can raise levels temporarily.

Your result is interpreted together with your symptoms, blood glucose, timing of your last meal, and any stimulation or suppression testing. If the result does not match how you feel or your glucose readings, your clinician may repeat the test, check related tests such as insulin, glucose, and antibodies, review medications, or evaluate kidney function.