C-peptide (finger-prick)

The beta cells of the pancreas first make proinsulin, which is cut into insulin and C-peptide in equal amounts. Unlike insulin, C-peptide is not broken down by the liver and stays in the blood longer; it is removed through the kidneys. Insulin medication contains no C-peptide. As a result, C-peptide shows how much insulin the pancreas makes itself, even in people who inject insulin. The value is always assessed together with the glucose measured at the same moment.

• How much insulin the pancreas still makes itself in known diabetes, for example when in doubt between type 1 and type 2 diabetes
• Cause of an unexplained low blood sugar: too much of your own insulin or administered insulin
• Rarely: an insulin-producing tumor of the pancreas (insulinoma)

• Do not eat anything for 12 hours before the sample is taken and drink only water. After a meal the value is many times higher.
• Do you take biotin (vitamin B8, often in hair and nail supplements)? Stop at least 2 days before the sample is taken, or 4 days for a high dose.

The GP assesses C-peptide together with glucose and HbA1c. With a low C-peptide and a high glucose, or with doubt about the type of diabetes, testing for antibodies against the pancreas often follows, along with consultation with or referral to an internist (specialist in internal medicine). With repeated unexplained low blood sugars, referral to an internist follows.

C-peptide is a reliable measure of your own insulin production, but can only be interpreted alongside the glucose at the same moment. It is not a suitable test for detecting diabetes: diabetes is diagnosed with fasting glucose, and in known diabetes HbA1c is monitored (the Dutch GP guideline (NHG) on type 2 diabetes mellitus). The result is most meaningful for a specific question: does someone with diabetes still make their own insulin, or what is the cause of an unexplained low blood sugar. To distinguish between type 1 and type 2 diabetes, C-peptide is combined with antibodies against the pancreas.

• Nutritional state: after a meal the value is much higher; a fasting blood draw makes the result comparable.
• The glucose at the same moment is needed to interpret the value; a single C-peptide value says little.
• Reduced kidney function raises the value, because C-peptide is removed through the kidneys.
• Sulfonylureas and other diabetes medicines change the value; measurement methods differ between laboratories.

In 2024, more than 1.1 million people with type 2 diabetes were known to their GP; type 1 diabetes is much less common. An unexplained low blood sugar in people without diabetes is rare, and an insulinoma is very rare.

The result is most meaningful in people with known diabetes in whom there is doubt about the type or about their own insulin production, and when investigating an unexplained low blood sugar. Not suitable for detecting diabetes: fasting glucose and HbA1c say more about that. In people without symptoms, a single value says little.

• A raised C-peptide without symptoms is quickly interpreted as disease, while glucose and HbA1c may be normal and the advice is then no different than for overweight.
• A non-fasting blood draw or reduced kidney function can give a falsely high result.

• NVKC, allesovertesten.nl: C-peptide
• NVKC, allesovertesten.nl: Insuline
• NHG-Standaard Diabetes mellitus type 2 (M01)
• Thuisarts.nl: Diabetes type 1
• Thuisarts.nl: Ik heb diabetes type 2 en mijn bloedsuiker is te laag