A C-peptide test is a blood test that measures levels of the protein fragment C-peptide in a person’s bloodstream. Because the body releases C-peptide in equal amounts to insulin in most cases, blood C-peptide levels can indicate how much insulin is circulating in the body.

The amount of C-peptide in a person’s bloodstream can indicate the presence or absence of a number of diseases and disorders. It may be used to establish the type of diabetes patients have after they have been diagnosed. The test may also be used to monitor a patient’s diabetes or other disorder.

The test usually requires a blood sample be taken from a vein (venipuncture) on the inside of the elbow or the back of the hand. Results are then compared to normal values. A blood glucose test may also be performed at the time of the C-peptide test and both test results may be compared.

When a disease or disorder is detected by the blood test, treatment for the condition usually follows.

Although the function of C-peptide is unknown, researchers are exploring the possibility that C-peptide therapy may decrease a patient’s risk of certain diabetic complications.