Patients with diabetes may get a C-peptide test as a means of distinguishing type 1 diabetes from type 2 diabetes or Maturity onset diabetes of the young (MODY). Measuring C-peptide can help to determine how much of their own natural insulin a person is producing as C-peptide is secreted in equimolar amounts to insulin. C-peptide levels are measured instead of insulin levels because C-peptide can assess a person's own insulin secretion even if they receive insulin injections, and because the liver metabolizes a large and variable amount of insulin secreted into the portal vein but does not metabolize C-peptide, meaning blood C-peptide may be a better measure of portal insulin secretion than insulin itself.

The Cardio IQ® Insulin Resistance Score (IR), which uses fasting insulin and C-peptide measurements to estimate a person's likelihood of having insulin resistance. It eliminates the need for multiple samples and reduces the chance of errors that can occur with some other tests.

Having a high score (indicating a greater probability of insulin resistance) was linked to an increased risk of developing type 2 diabetes, even after accounting for other risk factors. 

An IR score of <33 suggests that an individual has normal insulin sensitivity.
A score of 33 to 66 suggests that an individual has >4-fold greater odds of having IR compared with an individual with a score <33 .
A score >66 suggests that an individual has >15-fold greater odds of having IR compared to an individual with a score

Fasting Required for 10-12 hours. Water only.