What is c-peptide assay test in diabetes?

Measures insulin: The c-peptide is a by-product molecule that the pancreas makes when producing the Insulin hormone. Clinicians use this blood test measure to determine the Insulin production capacity from the pancreas, and to distinguish from Insulin injected as medication (since Insulin rx does not contain c-peptide). The Insulin assay test itself is also less preferable, due to variations in measurements.

Secreted Insulin: The pancreas beta cells make proinsulin, a u-shaped, inactive protein. On release of proinsulin, the beta cells cleave off the u connection; the protein is now active insulin. The split-out u connection, called c-peptide, is also released into the water around cells & filtered out by the kidneys. Thus blood level and/or urine output of c-peptide reflects amount proinsulin activated into insulin.