The fasting insulin test is a blood test that measures the level of insulin in your blood after you have fasted (not eaten or drunk anything except water) for at least 8 hours. Insulin is a hormone produced by the pancreas that helps regulate blood sugar levels. The fasting insulin test is often used to help diagnose conditions such as insulin resistance, prediabetes, and type 2 diabetes. It may also be ordered to monitor the effectiveness of treatment for these conditions. The test is usually done in the morning, after an overnight fast. The test results will be reported in units of insulin per liter of blood (U/L). It is important to note that the interpretation of a fasting insulin test may vary depending on the individual's medical history and other factors. A healthcare provider should be consulted to interpret the test results correctly.

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.