C-Peptide

C-peptide is a small protein released by your pancreas in exactly equal amounts to insulin. When your pancreas makes insulin, it first produces a larger molecule called proinsulin, which is then snipped into two pieces: active insulin and C-peptide. Because they are produced one-for-one, measuring C-peptide gives a reliable readout of how much insulin your pancreas is actually producing.

You might wonder why we would measure C-peptide rather than insulin directly. The answer lies in how the body handles each one. Insulin is rapidly cleared by the liver and has a short half-life, making blood levels fluctuate significantly. C-peptide, by contrast, is cleared more slowly and at a steadier rate, so it provides a more stable and accurate picture of pancreatic insulin production.

C-peptide is particularly valuable in distinguishing between different types of blood sugar conditions. In type 1 diabetes, where the insulin-producing cells in the pancreas are gradually destroyed, C-peptide levels are low or undetectable. In type 2 diabetes or insulin resistance, C-peptide is typically normal or raised because the pancreas is working overtime to produce extra insulin. This test can also help clarify whether someone with diabetes still has some residual insulin production, important information when deciding on the best treatment approach. In rare cases, it is used to investigate unexplained low blood sugar episodes, helping to distinguish between the body overproducing insulin and insulin being administered externally (injected insulin does not come with C-peptide). If your C-peptide falls outside the expected range, your GP or diabetes specialist can use it alongside your glucose and HbA1c results to understand your metabolic health more precisely and guide management decisions.