Did you know there is a lab test that can tell us how much insulin we’re making? It’s called a fasting c-peptide test and it can be ordered with any routine blood work.
Beta cells work in the pancreas to turn proinsulin into one-part insulin and one-part c-peptide. Because of this, c-peptide and insulin are made in equal amounts. It is therefore a useful marker for insulin production. Although this test is not used to diagnose diabetes, it is frequently ordered after diagnosis to find out how much endogenous (internal) insulin is being produced.
Type 1 diabetes is most often caused by an overactive immune response leaving the pancreas’ beta cells destroyed. Once complete destruction occurs, there is no production of insulin or c-peptide. Often after a fresh type 1 diagnosis, someone can still have some residual endogenous insulin production. This is what we call the “honeymoon phase” and can result in a positive c-peptide test in a type 1 patient. After honeymooning, we type 1’s make no insulin (or very very small amounts) and C-peptide becomes undetectable in our blood.
A person with diabetes can also have a high c-peptide…but how? People with type 2 diabetes and insulin resistance can actually make more insulin than people without diabetes. A high/normal c-peptide in a person with diabetes indicates that the diabetic is making plenty of insulin, they are just not responding correctly to it. Because of this, a c-peptide level is highly valuable when choosing drug therapies.
Someone with diabetes and a high/normal c-peptide will respond well to therapies like metformin, GLP-1’s, and SGLT2’s (and many more) however, someone with an undetectable c-peptide will require insulin (because they’re not making