C-Peptide Test

What are the other Names for this Test? (Equivalent Terms)

• Connecting Peptide Insulin Test
• Insulin C-Peptide Test
• Proinsulin C-Peptide Test 

What is C-Peptide Test? (Background Information)

• C-peptide is an inert stretch of amino acids formed during the production of insulin (more specifically, during its activation from proinsulin). This is done by the beta cells of the pancreas. Levels of C-peptide correlate with insulin production; thus, C-peptide serves as a marker for insulin production
• C-peptide is only created, when beta cells transform proinsulin to insulin. Injecting an outside source of insulin does not change C-peptide levels. The kidneys are responsible for eliminating C-peptide
• The C-Peptide Test is a blood or urine test that determines the levels of C-peptide in the body. This helps assess insulin production
• When this test is combined with an insulin test, it helps determine whether the excess insulin is due to native or endogenous insulin, which is made by the body from any insulin that is administered as a medication
  • An insulin/C-peptide ratio less than 1.0, indicates endogenous insulin secretion
  • An insulin/C-peptide ratio greater than 1.0, indicates an excess of exogenous insulin 

What are the Clinical Indications for performing the C-Peptide Test?

Following are the clinical indications for performing a C-Peptide Test:

• Distinguishing between insulin-dependent (type I) and non-insulin-dependent (type 2) diabetes
• Monitoring treatment and progression of insulinoma
• Monitoring health of transplanted pancreas
• Chronic sweating
• Altered mental status
• Blurred vision 

How is the Specimen Collected for C-Peptide Test?

Following is the specimen collection process for C-Peptide Test:

Sample required: Blood or urine

Process:

• Insertion of a needle into an arm vein, for a blood sample
• Collection of urine periodically over 24 hours, for a urine sample

Preparation required: No special preparation is needed prior to the test. 

What is the Significance of the C-Peptide Test Result?

The significance of C-Peptide Test is explained:

• Increased C-peptide levels may indicate:
  • Endogenous hyperinsulinism
  • Islet cell tumor
  • Non-insulin-dependent (type 2) diabetes
  • Oral hypoglycemic medication
  • Kidney disease or failure
  • Low blood potassium
  • Cushing’s syndrome
  • Lack of response to treatment of a tumor
• Decreased C-peptide levels may mean:
  • Factitious hypoglycemia
  • Insulin-dependent (type 1) diabetes
  • Response to treatment of a tumor 

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario. 

Additional and Relevant Useful Information:

• Certain factors may interfere with the results of the C-Peptide Test. These include:
  • Recent radioactive exposure, as with scans
  • Obesity
• The C-Peptide Test should be performed in the same laboratory each time, in order to ensure accurate results 

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.