C-peptide is a blood test used to assess how much insulin the body produces itself. The test can provide important information about the pancreas's insulin production and is used, among other things, in the investigation of diabetes, insulin resistance and recurrent low blood sugar.
C-peptide should not be interpreted as a stand-alone test. The result needs to be assessed together with the glucose level at the time of sampling and can also be supplemented with HbA1c, which shows how blood sugar has averaged over the past two to three months.
What does a C-peptide test show?
When the pancreas produces insulin, a precursor called proinsulin is first formed. Proinsulin is then divided into insulin and C-peptide, which are released into the blood in approximately equal amounts. By measuring C-peptide, it is therefore possible to estimate the body's own insulin production.
It is particularly valuable for people who are treated with insulin, since injected insulin does not contain C-peptide. The test can therefore help to distinguish the body's own insulin production from insulin supplied through drug treatment
When may it be relevant to measure C-peptide?
A C-peptide test may be relevant when there is a need to assess the pancreas's residual insulin production. The test can, for example, be used for:
- investigation of unclear diabetes type.
- assessment of residual insulin production in diabetes.
- suspected insulin resistance and high insulin production.
- decision on insulin treatment or other diabetes treatment.
- follow-up of people with type 1 or type 2 diabetes.
- recurrent episodes of low blood sugar.
- Suspected overproduction of insulin.
- Investigation of a rare insulin-producing insulinoma.
How is C-peptide interpreted?
The interpretation depends on the C-peptide value, the simultaneous glucose value, whether the sample was taken fasting or after a meal, as well as the person's kidney function, medical background and any diabetes treatment.
A C-peptide value cannot therefore be assessed solely as high, normal or low without taking into account how high the blood sugar was when the sample was submitted.
What can a low C-peptide value mean?
A low C-peptide value in combination with elevated glucose may indicate that the pancreas is producing too little insulin. This can occur, for example, in type 1 diabetes or in advanced type 2 diabetes, when the function of the insulin-producing beta cells has deteriorated.
However, a low value alone is not sufficient to diagnose type 1 diabetes. If the type of diabetes is uncertain, the investigation may also need to be supplemented with autoantibodies, such as GAD antibodies and IA-2 antibodies.
Long-term, severely elevated blood sugar can also temporarily inhibit the pancreas's insulin production. A low value may therefore sometimes need to be checked again when the blood sugar has stabilized.
What can a high C-peptide value mean?
A high C-peptide value means that the body is producing a lot of its own insulin. This can occur in insulin resistance, when the body's tissues respond less well to insulin and the pancreas compensates by increasing insulin production. Elevated levels can be seen in obesity, metabolic syndrome and early type 2 diabetes, among other things. C-peptide can also be affected by impaired kidney function, as the substance is largely broken down and excreted via the kidneys.
In the case of low glucose in combination with an unexpectedly high or measurable C-peptide value, further medical investigation may be needed to assess whether the body is producing too much insulin.
C-peptide together with glucose and HbA1c
C-peptide, glucose and HbA1c measure different parts of the body's blood sugar regulation and therefore complement each other.
- C-peptide provides an estimate of the body's own insulin production.
- Glucose shows the blood sugar level at the time of sampling.
- HbA1c, also called long-term sugar, shows the average blood sugar over the past two to three months.
C-peptide should be analyzed together with glucose, since a C-peptide value needs to be put in relation to the current blood sugar level. The analysis can also be combined with HbA1c – long-term sugar for a broader picture of blood sugar regulation.
Can C-peptide diagnose diabetes?
C-peptide is not primarily used to determine whether a person has diabetes. When diabetes is suspected, HbA1c, fasting glucose or glucose load are primarily used to assess whether blood sugar meets the criteria for diabetes. C-peptide is instead used as a supplementary test to assess insulin production and can contribute to classifying the type of diabetes or choosing treatment.
Do I need to fast before a C-peptide test?
C-peptide can be analyzed both fasting and in connection with insulin production being stimulated by food. Which sampling situation is most relevant depends on the question and the laboratory's sampling routines. Since food affects both insulin, C-peptide and glucose, it is important that the test result is interpreted based on whether the sample was taken fasting or after a meal. Always follow the instructions provided before taking the sample.
When is medical assessment needed?
An abnormal C-peptide value should be assessed together with symptoms, glucose, HbA1c, kidney function and any diabetes treatment. If diabetes is suspected, further analyses may be needed to confirm the diagnosis and type of diabetes. If symptoms such as pronounced thirst, large amounts of urine, rapid weight loss, nausea or clear general effects occur, healthcare should be contacted urgently. C-peptide is not intended to rule out acute diabetes on its own.
