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What is a C-peptide test?

A C-peptide test is used to estimate the amount of C-peptide present in the blood. C-peptide is a short protein produced by the pancreas. It is released at the same time and in an equal quantity as that of insulin (the hormone responsible for metabolising glucose) but stays in the blood much longer than insulin; thus, a C-peptide test helps in evaluating the amount of insulin made by the body.

This test also helps in differentiating body’s own insulin from the external sources of insulin (as a diabetic medicine), which does not generate C-peptide. It is usually performed along with an insulin test.

  1. Why is a C-peptide test performed?
  2. How do you prepare for a C-Peptide test?
  3. How is a C-peptide test performed?
  4. What do C-peptide test results indicate?

A C-peptide test is useful in distinguishing between type 1 and type 2 diabetes. It is also helpful in determining the cause of hypoglycaemia (low blood sugar levels), checking the efficiency of diabetic drugs (oral hypoglycaemic drugs) and evaluating pancreatic tumours.

C-peptide levels are usually checked in people who experience frequent episodes of hypoglycaemia. The symptoms of hypoglycaemia include:

  • Profuse sweating
  • Palpitations (rapid or irregular pulse)
  • Abnormal hunger
  • Blurring of vision
  • Confusion
  • Fainting episodes
  • Loss of consciousness or seizures in severe cases 

In patients with type 2 diabetes, C-peptide is also used as a routine test to evaluate the functioning and efficiency of beta cells. Although both insulin and C-peptide are produced in equal quantities by beta cells, the half-life of insulin is 5 minutes, while that of C-peptide is about 30 minutes, which makes C-peptide test more reliable for evaluating insulin production than insulin test. This test also helps in determining whether insulin injections are required.

Additionally, a C-peptide test helps in the diagnosis of insulinoma, periodic monitoring of the efficiency of insulinoma therapy and it also aids in checking for recurrence of the condition.

In patients who have undergone pancreas islet cell transplants, C-peptide levels help in monitoring the functioning of these transplanted cells.

Eight to twelve hours of fasting is required before collecting blood for a C-peptide test.

It is a simple test that takes less than five minutes. An experienced laboratory specialist will collect a blood sample from a vein in your arm by inserting a small needle. The sample will be collected into a sterile vial or a test tube.

A momentary pricking pain is felt when the needle goes in the vein and there is a minimal risk of light-headedness and bruising at the site of injection. However, at most times, these symptoms disappear quickly. Rarely, an infection may occur at the site of withdrawal of blood

In certain situations, urine C-peptide levels are also evaluated. For this, twenty-four-hour urine samples are collected (all the urine that is produced by the body is collected over a period of 24 hours).

C-peptide levels are evaluated as nanograms per millilitre (ng/mL) or expressed in nanomoles per litre (nmol/L).

Normal results: Normal level of C-peptide ranges from 0.51 to 2.72 ng/mL (0.17-0.9 nmol/L).

Abnormal results:

High values:

Higher than normal values of C-peptide indicate increased production of endogenous insulin that can occur in the following conditions:

  • Hyperglycaemia due to insulin resistance
  • Insulinomas (tumour of the beta cells of the pancreas)
  • Low blood potassium (hypokalaemia)
  • Cushing’s syndrome
  • Renal failure
  • Type 2 diabetes

Low values:

Lower than normal values of C-peptide indicate a reduction in endogenous insulin production, which can happen in conditions, such as

During the monitoring of insulinomas, a decreasing level of C-peptide indicates a good response to the therapy, while increasing levels indicate poor response to therapy or tumour recurrence.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational perspective and is in no way a substitute for medical advice from a qualified doctor.

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  1. MedlinePlus Medical Encyclopedia: US National Library of Medicine; C-Peptide Test
  2. Seok Man Son. C-Peptide and Vascular Complications in Type 2 Diabetic Subjects. Diabetes Metab J. 2012 Oct; 36(5): 345–349. PMID: 23130318
  3. Carl Burtis Edward Ashwood David Bruns. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 5th Edition; ISBN: 9781455777112
  4. Kathleen Pagana Timothy Pagana. Mosby's Diagnostic and Laboratory Test Reference. 10th Edition; ISBN: 9780323168823
  5. [internet] Diabetes Digital Media Ltd; C-peptide Test.