The C-Peptide Test
Reviewed by CDE Mehandi Sharma · Updated
C-peptide is a small piece of protein released into your blood every time your pancreas makes insulin, in equal amounts. Measuring it tells you how much insulin your own body is still producing.
This is useful in a way that measuring insulin is not. Injected insulin contains no C-peptide, so the test still reflects only what you make yourself even if you are already on insulin.
A typical normal range is around 0.8 to 3.0 ng/mL, though ranges differ between laboratories.
| Condition | Typical C-peptide |
|---|---|
| Type 1 diabetes | Very low or undetectable |
| LADA | Low, and falling over time |
| Type 2 diabetes | Normal or high |
That middle position is what makes the test so useful in LADA. It is also why a single reading is not the whole story — what matters is the direction of travel, so the test is often repeated over the years.
There are two ways to do it. A fasting sample is simpler. A stimulated sample, taken after a measured drink or meal, shows how much insulin your pancreas can produce when it is asked to, and predicts more reliably when insulin will be needed.
C-peptide is cheap and widely available, which makes it a sensible first step — but it cannot confirm LADA on its own. Only the antibody test does that.