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Fasting C-Peptide — Protocol

Published: August 9, 2026Updated: August 16, 2026

A fasting blood test that measures C-peptide, a substance released with insulin, to help evaluate the body's own insulin production.

Quick summary

["Fasting C-peptide is usually interpreted with glucose and insulin levels to assess insulin production and secretion."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
62803-2
Short code
Fasting C-peptide proto
Synonyms
Fasting C-peptide proto
Category
liver
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
40
Critical range
50 N/A – 400 N/A
Fasting
true
Population
adult
Sex
all
Age range
18–65 years
Method
phenx
CPT
84484
SNOMED
302774002
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What is this test?

C-peptide is made by the pancreas when it releases insulin. This test measures the amount of C-peptide in a blood sample after a period of fasting, providing information about the body's own insulin secretion.

Why might I need this test?

A clinician may order this test when evaluating diabetes, insulin resistance, unexplained low blood sugar, or other concerns about insulin production. It is often reviewed together with fasting glucose and insulin measurements.

How is the test performed?

A healthcare professional draws blood from a vein, usually after the required fasting period. The sample is sent to a laboratory, where C-peptide is measured and the results are reported with your other diabetes test results.

How do I prepare for the test?

Fasting for this test is required. Follow the fasting instructions provided by your healthcare professional, including when to stop eating and whether water and medications are permitted. Tell the clinician about insulin injections or other medicines that can affect the results.

Reference range

Adult: 10-40 U/LN/A

Specimen: serum

Fasting for 8-12 hours required. Water is permitted. Avoid chewing gum and smoking before the test.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Fasting C-Peptide

Frequently asked questions

Does the test measure insulin itself?
No. It measures C-peptide, which is released by the pancreas when insulin is produced. C-peptide can help assess endogenous insulin secretion, especially when considered with insulin and glucose tests.
How long do I need to fast?
The required duration depends on the laboratory protocol and the reason for testing. Follow the specific instructions from your healthcare professional, and ask if you are unsure whether medications can be taken.
Can insulin injections affect the result?
Yes. Injected insulin does not contain C-peptide, and insulin treatment can complicate interpretation of the result. Tell your clinician about all insulin and other diabetes medicines before testing.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy