C-peptide is released by the pancreas in equal amounts with insulin, but it lasts longer in the blood and isn't affected by insulin injections. That makes it a steadier read on how much insulin your pancreas is actually producing.
High fasting C-peptide points to the pancreas working overtime to control blood sugar — an early sign of insulin resistance, often before glucose or A1c rise. Low C-peptide suggests the pancreas is producing too little insulin. Read together with fasting insulin, glucose and HOMA-IR.
Optimal C-Peptide: 1-2 ng/mL.
Typically re-tested every 12 weeks.