If you have type 1 diabetes, your doctor may test your c peptide level to see how much insulin your body is producing. This can help your doctor decide if you need insulin injections, and when. C peptide is an important part of the insulin hormone that decreases blood sugar (glucose) levels by transporting glucose from the bloodstream into cells and tissues for use as energy.
In general, normal c peptide levels are less than 2 nanograms per milliliter of blood.
The c peptide test can be done in your doctor’s office or at a lab. A person in the doctor’s office or lab will place a needle into your vein and draw a small amount of blood. This can hurt slightly, but is not dangerous.
You will need to fast before the test, meaning you should not eat anything but water. If you have a c peptide urine test, you will collect some of your urine in a container and return it to the lab.
C peptide results vary from laboratory to laboratory. This is because different assay methods are used. Standardization of these assays is needed to achieve comparable results between research and clinical laboratories.
In general, lower c peptide levels are associated with poorer glycemic control and increased HbA1c. However, this correlation needs to be evaluated in the context of the duration and age of diagnosis of diabetes, comorbidities, family history, and insulin resistance. Additionally, longitudinal c peptide measurements may be useful in the prediction of rare forms of diabetes such as MODY and LADA.