C-Peptide Test
A C-peptide test measures C-peptide, a substance the pancreas releases alongside insulin as it makes the hormone. Because it is produced in step with the body's own insulin, clinicians use it to reflect the body's own insulin production, read together with other markers.
About this test
When the pancreas makes insulin, it first produces a larger molecule that is split into two parts: insulin itself and a connecting fragment called C-peptide. Both enter the bloodstream together, so C-peptide travels alongside the insulin the body has produced.
The C-peptide test is done with a simple blood draw at a patient service center. A phlebotomist collects a sample from a vein, usually in the arm, and the sample goes to the laboratory. Results are read by a clinician in the context of symptoms, history, and any other markers ordered alongside it.
Who might consider this test?
- People who order a C-peptide test are often interested in how much insulin the body is producing on its own.
- Taking a closer look at their metabolic health
- Following up on questions raised by other markers
Test details
| Quest code | 372 |
|---|---|
| Methodology | Immunoassay |
| Sample type | 1 mL serum |
| Patient preparation | Fast for 12 hours before your blood draw. |
| Collection | Quest patient service center |
Reference range
0.80-3.85 ng/mL
Reference ranges are general guidance. The range printed on your own laboratory report is the one that applies to your result.
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Additional details
What this test looks at
C-peptide reflects the insulin the body makes itself. Since it is released in step with the body's own insulin, a C-peptide test can offer a sense of that internal production that a direct insulin measurement does not always capture in the same way.
It also helps to remember what C-peptide is not: on its own, a single value is a piece of a larger picture rather than an answer. That is exactly why a clinician reads it alongside glucose, insulin, and the person's circumstances rather than in isolation.
How it's often used alongside other tests
C-peptide is frequently considered together with insulin, since the two describe closely related aspects of the same process — C-peptide reflecting the body's own production and insulin reflecting the hormone circulating in the blood. A clinician may also weigh it against glucose and hemoglobin A1c, which describe the sugar side. Read together, these markers offer a fuller sense of how the body is producing and using insulin than any single one can on its own.
Order online and a physician order is included — no doctor's visit and no insurance needed. Your sample is collected at a Quest Diagnostics patient service center.
Laboratory results should always be interpreted by a licensed clinician who knows your health history. This page is information about a test, not medical advice.
Frequently asked questions
What does a C-peptide test measure?
It measures C-peptide, released by the pancreas alongside insulin as the body makes it, so it reflects the body's own insulin production. A clinician reads it in the context of glucose, insulin, and your history.
How is a C-peptide test different from an insulin test?
An insulin test measures the hormone that regulates blood sugar, while a C-peptide test reflects the body's own insulin production specifically, since C-peptide comes only from insulin the body makes itself.
Do I need to prepare for this test?
Fast for 12 hours before your blood draw. This is the preparation Quest publishes for this test; if your clinician gives you different instructions, follow theirs.
How is the sample collected?
Through a routine blood draw at a patient service center. A phlebotomist takes a small sample from a vein in the arm and sends it to the laboratory.
Does a C-peptide test diagnose a blood sugar condition?
No single test diagnoses a condition. A C-peptide result is one piece of information a clinician considers along with glucose, insulin, your history, and exam to help evaluate how the body produces and regulates insulin.
How much is a C-Peptide test?
$119, paid once at checkout. The price includes the physician's order this test requires and sample collection at a Quest patient service center.