Related tests: Insulin, Glucose, anti-insulin antibodies

Clinical significanceClinical indicationsSample typePreparationNotes

Clinical significance
C-peptide comes from the breakdown of proinsulin inside the beta cells of the pancreas. Once this breakdown has taken place, equimolar amounts of insulin and C-peptide are released into the portal circulation. The advantage of measuring C-peptide rather than insulin is that it reflects the true state of beta cell secretion more accurately. Once released into the portal circulation, more than 50% of insulin is taken up by the liver on its first pass and it is also significantly taken up by target tissues. Blood insulin therefore does not fully reflect the endocrine function of the pancreatic beta cells.
C-peptide, on the other hand, is only minimally taken up by the liver (about 10%), and a further advantage is its half-life, at least twice that of insulin, which makes serum levels more stable. Most circulating C-peptide is broken down in the kidneys and partly excreted in the urine.
C-peptide is raised in the following conditions: insulinoma, kidney failure, non-insulin-dependent (type 2) diabetes and taking oral diabetes medicines.
C-peptide is reduced in the following conditions: factitious hypoglycaemia due to insulin administration, radical pancreatectomy and insulin-dependent (type 1) diabetes.

Clinical indications
In serum, the test is particularly useful for assessing residual beta cell function in diabetic patients treated with injected insulin. In these cases serum insulin cannot be measured, because the anti-insulin antibodies constantly present in the serum of people with diabetes make it impossible to assess insulin. In 24-hour urine, the test is useful when beta cell function needs to be assessed continuously.

Sample type
The patient must have a blood and/or urine sample taken.

Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes. For the urine test, collect a 24-hour urine sample.

Notes
C-peptide can be raised by chloroquine and oral contraceptives.