After a diabetes diagnosis, it may be necessary to identify the ability of the pancreas to produce insulin in order to determine the exact type of diabetes as well as its treatment.
Testing the level of insulin in the blood is not normally used to establish the ability of the pancreas to produce insulin. The reason is that insulin first travels from the pancreas to the liver, with only about half of it distributed from there to the rest of the body by blood. Of the endogenous insulin created by beta cells, a part called C-peptide is removed, leaving the active insulin.
Insulin production can be determined by simultaneously testing the blood sample for the blood C-peptide and glucose level, i.e. blood sugar. Insulin deficiency can be analysed on the basis of the C-peptide level only if the blood sugar level is high i.e. > 7 mmol/l. If the blood sugar level is lower than 7 mmol/l, the C-peptide level is normally low in any case.
In the case of insulin deficiency, the blood sugar level is high and the C-peptide level is low. In the case of insulin resistance, on the other hand, the C-peptide level is high. In insulin resistance, the effect of insulin in the tissues is reduced and insulin production in the pancreas is increased.
When is C-peptide measured?
C-peptide is measured if there is a need to determine the exact degree of insulin deficiency. It may be necessary in order to determine the type of diabetes or assess the status and mode of treatment of a person who has had diabetes for a longer period of time.
Determining the C-peptide level does not have an effect on the choice of the initial treatment. At the early stages of diabetes, interpreting the C-peptide levels may be difficult because long-term high blood sugar in itself reduces insulin production, which means that the C-peptide level may be temporarily reduced.
Interpreting the C-peptide results
The C-peptide level and blood sugar are usually measured after breakfast. The meal increases blood sugar and accelerates insulin production. Two hours after the meal and when the blood sugar level is over 7 mmol/l, the C-peptide level is normally over 0,7 nmol/l (nanomol/litre). In the case of diabetes, level below 0,7 nmol/l is a sign that insulin is required and level below 0,3 nmol/l is a sign of significant insulin deficiency.
On the other hand, a high C-peptide level of 0,9 nmol/l is a sign of reduced insulin efficacy and insulin resistance.