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. Because C-peptide comes only from insulin the body makes itself, a C-peptide test offers a view of the pancreas's own insulin output, seen from a slightly different angle than an insulin test.
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. Some C-peptide tests call for a period of fasting beforehand, so people follow any instructions their ordering clinician provides. Results are read by a clinician in the context of symptoms, history, and any other markers ordered alongside it.
Who commonly orders it
People who order a C-peptide test are often interested in how much insulin the body is producing on its own. Some are taking a closer look at their metabolic health, following up on questions raised by other markers, or exploring this at a clinician's suggestion. Because C-peptide describes the body's own insulin production, it is frequently considered alongside insulin and glucose so a clinician can view the hormone side of blood sugar regulation from more than one angle.
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. Like insulin, C-peptide is closely tied to timing and circumstances, so whether a sample was taken while fasting is part of what gives the result meaning.
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.
What to expect at your visit
People visit a patient service center during open hours, and the draw itself is quick — a phlebotomist collects a small sample from a vein in the arm, and most visits wrap up in a few minutes. Because some C-peptide tests ask for a period of fasting, it is worth checking the instructions for the test you order ahead of time. Once the laboratory completes its analysis, results are posted to the account, where they can be reviewed and shared with a clinician.
Your results
Results will be emailed securely to you as a PDF.
One-time price · physician order included · no insurance needed
Prefer a fuller panel?
A curated bundle groups this test with the markers clinicians most often review alongside it — often better value than single tests.
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 fast before a C-peptide test?+–
Some C-peptide tests call for a period of fasting because production varies with eating and timing. It is worth checking the instructions for the specific test you order and following any guidance your ordering clinician provides.
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?+–
$125, paid once at checkout. The price includes the physician's order this test requires and sample collection at any of 2,300+ Quest patient service centers nationwide.