Fasting C-Peptide, Quantitative — Assessment of Residual Beta-Cell Function
A fasting blood test that measures C-peptide, a marker of the body's own insulin production and pancreatic beta-cell function.
Fasting C-peptide helps show how much insulin your pancreatic beta cells are producing when you are not taking insulin.
Key facts at a glance
- LOINC
- 62802-4
- Short code
- PHENXMEASUREFASTINGCPEPTIDEASSAYFORRESIDUALBETACELLFUNCTION
- Synonyms
- —
- 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 released by pancreatic beta cells at the same time as insulin. Measuring fasting C-peptide helps assess the body's own insulin production and residual beta-cell function.
Why might I need this test?
This test may be used to evaluate unexplained low blood sugar, investigate unusually high or low insulin levels, or assess remaining pancreatic beta-cell function in diabetes. It is often interpreted with blood glucose and insulin measurements.
How is the test performed?
A blood sample is collected, usually after an overnight fast, and C-peptide is measured in the laboratory. Results are commonly reviewed alongside your current medications, blood glucose, and insulin level.
How do I prepare for the test?
Follow your clinician's fasting instructions; this commonly means no food or drinks other than water for at least 8 hours. Do not stop or change diabetes medications unless your clinician specifically instructs you to do so.
Reference range
Adult: 10-40 U/LN/A
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.
A higher-than-expected C-peptide level may indicate continued or increased production of the body's own insulin, including in some cases of insulinoma or after taking certain diabetes medications. A lower level may suggest reduced pancreatic beta-cell function. Interpretation depends on blood glucose, insulin, symptoms, medications, and other clinical findings.
Short name
Fasting C-Peptide
Frequently asked questions
Why is C-peptide measured when insulin is also measured?
Does a low C-peptide result always mean I have type 1 diabetes?
Can I take my usual medications before this test?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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