When a patient has newly diagnosed type 1 or type 2 diabetes, C-peptide can be used to help determine how much insulin the patient’s pancreas is still producing.

Type 1 diabetes is an autoimmune process that often starts in early childhood and involves the almost complete destruction of the beta cells over time.  Eventually, little or no insulin (and C-peptide) is produced, leading to a complete dependence on exogenous insulin.

In type 2 diabetes, there is insulin resistance and a compensatory increase in insulin production and release that can also lead to beta cell damage. Type 2 diabetics usually are treated with oral drugs to stimulate their body to make more insulin and/or to cause their cells to be more sensitive to the insulin that is already being made.  Eventually, because of the beta cell damage, type 2 diabetics may make very little insulin and require injections.  Any insulin that the body does make will be reflected in their C-peptide level; therefore, the C-peptide test can be used to monitor beta cell activity and capability over time and to help your doctor determine when to begin insulin treatment.

C-peptide measurements also can be used in conjunction with insulin and glucose levels to help diagnose the cause of documented hypoglycemia and to monitor its treatment.  Symptoms of hypoglycemia may be caused by excessive supplementation of insulin, alcohol consumption, inherited liver enzyme deficiencies, liver or kidney disease, or insulinomas (tumors of the islet cells in the pancreas that can produce uncontrolled amounts of insulin and C-peptide).

C-peptide levels may be ordered if you have newly diagnosed type 1 diabetes, as part of an evaluation of your “residual beta cell function” (how much insulin your beta cells are making).  With type 2 diabetes, the test may be ordered if your doctor wants to monitor the status of your beta cells and insulin production over time and to determine if/when insulin injections may be required. 

C-peptide levels may be done when there is documented acute or recurring hypoglycemia.  Symptoms include sweating, palpitations, hunger, confusion, blurred vision, fainting, seizures, and even loss of consiousness, although these symptoms also can occur with other conditions.  The C-peptide test may be used to help separate excessive insulin production from excessive administration and to help diagnose insulinomas.

If you have had your pancreas removed or are one of the few patients to have had pancreas islet cell transplants (in order to restore your ability to make insulin), your C-peptide levels may be monitored to verify the effectiveness of treatment and continued success of the procedure.

High levels of C-peptide generally indicate high levels of endogenous insulin production.  This may be a response to high levels of blood glucose caused by glucose intake and/or insulin resistance.  (With insulin resistance, the body’s cells do not use insulin normally to transport glucose inside the cell.  The cells become “starved for glucose,” interpret that as a lack of insulin, and signal the body to make more.)  High levels of C-peptide also are seen with insulinomas (insulin-producing tumors) and may be seen with hypokalemia, pregnancy, Cushing’s syndrome, and renal failure. 

Low levels of C-peptide are associated with low levels of insulin production. This can occur when insufficient insulin is being produced by the beta cells or when production is suppressed by exogenous insulin or with suppression tests that involve substances such as somatostatin.  

C-peptide testing is not widely used and may not be available in every laboratory.  If you are going to have a series of C-peptide tests performed, they should be done at the same laboratory using the same method.

Even though they are produced at the same rate, C-peptide and insulin leave the body by different routes.  Insulin is processed and eliminated mostly by the liver, while C-peptide is removed by the kidneys.  Since the half-life of C-peptide is about 30 minutes to insulin’s 5 minutes, normally there will be about 5 times as much C-peptide in the bloodstream as insulin.