C-peptide is the fragment released in equal amount to insulin when proinsulin is split inside the beta cells of the pancreas. Unlike insulin, it is not removed by the liver on first pass and is not present in injected insulin preparations, so it reflects the body's own secretion. It speaks for residual beta-cell function.
No readings for this marker yet.
Raised values most often accompany insulin resistance, obesity and type 2 diabetes, where the pancreas secretes generously, and they also rise with drugs that stimulate secretion, such as gliclazide. Low values point to reduced or exhausted beta-cell output, as in type 1 diabetes and advanced type 2 diabetes. The measurement is most informative when the glucose drawn at the same moment is known, since low secretion is expected when glucose is low. Interpretation belongs with a doctor who knows the clinical picture and the treatment.
The sample is usually taken fasting, and food or glucose-lowering drugs taken beforehand shift it. C-peptide is cleared by the kidneys, so impaired renal function raises it independently of the pancreas. It is read alongside a simultaneous glucose, and with insulin, HOMA-IR and HbA1c when the question is how much secretion remains.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.