C Peptide (Fasting)
A C-Peptide (Fasting) test measures the amount of C-peptide in a sample of your blood after you have refrained from eating or drinking (except water) for a specific period, usually 8 to 12 hours.
C-peptide is a byproduct created when the pancreas produces insulin. Insulin and C-peptide are released into the bloodstream in equal amounts. Therefore, measuring C-peptide is an accurate, indirect way to see exactly how much insulin your pancreas is naturally producing.
Parameters Measured
Preparation
What a C-Peptide Test Can Reveal About Insulin Production
Insulin gets all the attention when it comes to blood sugar, but there's a quieter marker doctors rely on when they want to know exactly how much insulin your pancreas is producing on its own: C-peptide.
Here's the simple version. When your pancreas makes insulin, it also produces C-peptide as a byproduct, and it releases both into your bloodstream in roughly equal amounts. Insulin gets used up and broken down fast, which makes it a moving target to measure accurately. C-peptide sticks around longer and doesn't get affected by insulin injections, which makes it a more stable, reliable way to check your pancreas's actual output.
That last point matters a lot for anyone who's already on insulin therapy. A regular insulin test can't tell the difference between insulin your body made and insulin from a shot. C-peptide can, since it's only produced naturally.
Doctors typically reach for this test when they're trying to sort out the type of diabetes someone has, checking whether a diabetic's pancreas is still producing insulin, or investigating unexplained low blood sugar (hypoglycemia). It also comes up when insulin-producing tumours, called insulinomas, are suspected.
Since this is the fasting version of the C-peptide test, you'll need to avoid food and drink (other than plain water) for about 8 to 12 hours before your blood is drawn. This matters because eating triggers insulin release, which throws off the reading. A quick blood draw is all it takes, and results are typically available within 24 hours.