When insulin is first produced it is in the form of proinsulin and consists of chains of active insulin connected by an inactive amino acid called C-peptide.
Before it enters the blood it separates into two, the active insulin and the inactive c-peptide.So for every molecule of insulin in the blood, there is one of c-peptide.
But insulin does not stay in the blood for long, it is taken up by the liver. C-peptide, on the other hand, hangs around, therefore measuring levels of c-peptide in the blood is an indirect test of the amount of insulin the pancreas is producing.
What is the purpose of this test?
1. To determine how much insulin the patient's pancreas is still producing.
2. To determine how much Insulin Resistance you have.
3. To help determine the cause of hypoglycaemia
4. Distinguish between type 1 or type 2.
5. Monitor insulin production after the removal of a non-cancerous insulin-producing tumour of the pancreas (insulinoma).
How is this test done?
After fasting for 8-12 hours a blood sample is drawn and sent off to the lab.
Because some diabetes medications can interfere with the results of this test your doctor may tell you to stop taking them for a period before the blood is taken.
This particularly applies to Type 2's.
How to read the results.
'Normal' C-Peptide levels vary from one lab to another.
Normal Fasting = 0.17-0.83 micromoles per liter (mol/L).
In any case to properly interpret the results the C-peptide test results and the blood glucose test results must be considered together.
1. Undetectable C-peptides with high blood glucose indicates Type 1 diabetes.
2. High levels of both C-peptide and blood glucose are present then type 2 diabetes or insulin resistance is likely.
3. High level of C-peptide with a low blood glucose level may indicate an insulinoma, a pancreatic tumour that causes excess amounts of insulin to be produced. OR...
4. A high level of C-peptide with a low blood glucose level may be related to the use of certain medications such as sulfonylureas or meglitinides.
5. Low c-peptide level and low blood sugars (hypoglycaemia) can mean excessive use of synthetic insulin. OR..
6. Low levels of C-peptide and low blood glucose may be associated with liver disease. OR..
7. Low levels of C-peptide and low blood glucose may be caused by some form of severe infection. OR..
8. Low levels of C-peptide and low blood glucose may be caused by Addison's disease.
Factors which can cause inaccurate results:-
1. Medications. These include those used for diabetes, such as insulin & sulfonylureas. Corticosteroids, Oral contraceptives, Diuretics, Deferoxamine (a chelating agent used to treat iron poisoning and certain typed of anaemia), Rifampin (an antibiotic used to treat TB and carriers of meningitis), Terbutaline (used to treat asthma), Propranolol (a beta blocker used in the treatment of high blood pressure).
3. Renal failure.
4. Obesity - because of increases insulin production.
5. A radioactive scan within one week of a C-peptide test.
In Europe, clinical trials of C-peptide therapy are being conducted with promising results.
Type 1 diabetics show decreased diabetic complications, improvements in kidney function, blood flow, and nerve function.
This is however still very new and much more testing and trials need to be done.