C-peptide

For a fasting sample; many laboratories print a separate, higher range for a sample taken after food or a glucose drink, and some print ng/mL (1.1 to 4.4 for this range). Urine C-peptide is a different test with its own units.

C-peptide is a small protein released by the pancreas in equal amounts with insulin, one molecule of each, as insulin is made. Unlike insulin itself it is not affected by insulin injections, so measuring it shows how much insulin the pancreas is still producing on its own.

Doctors request it mainly in people who already have diabetes, to see how much insulin the pancreas is still making and so to help decide which type of diabetes it is. It is also used when looking into unexplained low blood sugar.

• Food or a glucose drink before the test
• Being overweight and insulin resistance
• Type 2 diabetes in its earlier years
• Kidney disease, because C-peptide is cleared by the kidneys
• Rare tumours that make insulin
• Some diabetes tablets that stimulate the pancreas

• Type 1 diabetes, where the pancreas makes little or none
• Long-standing type 2 diabetes
• A prolonged fast
• Diseases of the pancreas, including after surgery to remove part of it
• Insulin injections, because they lower the pancreas's own output for a while

• Whether the sample was fasting or after food changes the figure several-fold
• Reduced kidney function raises the figure without any change in the pancreas
• Methods and units differ between laboratories

Usually a fasting sample with a glucose taken at the same time; some tests are deliberately taken after a meal. The value is always read alongside the glucose from the same moment.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.