C-Peptide Test: What does it mean? And why would a Type 1 diabetic need it?

What is C-peptide?

C-peptide is a pre-cursor of insulin. Beta cells in our pancreas make insulin, and at the same time they release C-peptide molecules too. For every molecule of insulin that our body produces, one molecule of c-peptide is released. C-peptide on its own does not have an effect on blood glucose, but insulin does.

The C-peptide test is used to identify how much insulin our body produces. Doctors can use it to diagnose Type 1 diabetes and to help them identify if the patient has Type 1 or Type 2 and insulin resistance. C-peptide is a more accurate identification of our body’s insulin production than insulin level tests, as it remains in our blood for a longer period of time than insulin does. You can do a blood test for insulin or insulin (SI), but from that test you cannot differentiate between the injected (pumped) insulin and the insulin actually made by your own pancreas.

Why might we need C-peptide tests for Type 1 diabetics?

Some T1Ds may still produce a small amount of insulin. This will affect their blood glucose management and their requirements for insulin. Doctors are able to give better advice, help avoid hypoglycaemia as much as possible and advise more effective blood glucose management strategies, if they know that the patient’s pancreas is still producing some insulin.

Insulin production in Type 1 is especially common during the honeymoon period, which can last from a few months up to a few years. The earlier T1D is diagnosed and the more measure to maintain insulin production are taken, the longer the patient will remain in the honeymoon phase.

How is the C-peptide test done?

The test is a simple blood test, where blood is taken and analysed in a laboratory. It is usually done in a fasted state, and you are asked not to eat or drink sugary drinks or juices. Sometimes, the test is taken after a meal, depending on what the doctor wants to find out about our insulin production. Nothing to worry about, just a regular blood test.

How to understand C-peptide test results?

The normal C-peptide test results for an individual without Type 1 diabetes are 0.5 - 2.0 ng/ml. These will not be the same for T1Ds, as most of us do not produce insulin and are likely to not have the c-peptide molecules in our blood. If your test tells you that there is some c-peptide in your blood (and you are Type 1), it means that you are still producing small amounts of insulin.

Having insulin produced by the beta cells, when you have Type 1, can either make your glucose management more challenging or easier. It depends on whether your body is randomly producing insulin when it wants to, or whether your signalling pathways for insulin production are working properly and it is producing it in response to rising blood glucose levels. As said above, production of insulin in T1D is common during the honeymoon period, but it can be different for everyone in terms of whether it helps or complicates blood glucose management.

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