What is the role of C-peptide in differentiating diabetes types?

• A. C-peptide reflects endogenous insulin production; low or absent in Type 1; normal to high in early Type 2; helps differentiate.
• B. C-peptide is a marker of autoimmunity.
• C. C-peptide directly lowers blood glucose.
• D. C-peptide measures liver function.

Answer: (A)

The key idea is that C-peptide shows how much insulin the body is still making on its own. When proinsulin is split, insulin and C-peptide are released in equal amounts, and C-peptide isn’t cleared by the liver, so it serves as a reliable marker of endogenous insulin production.

In autoimmune Type 1 diabetes, the beta cells are destroyed, so endogenous insulin production drops to very low or undetectable levels, leading to low C-peptide. In early Type 2 diabetes, the pancreas often still produces insulin to overcome insulin resistance, so C-peptide levels are normal to high. Because of this, C-peptide helps distinguish between those who lack insulin production and those who still have it, which is especially useful in ambiguous cases or when patients are on insulin therapy.

C-peptide isn’t a marker of autoimmunity, it doesn’t directly lower blood glucose, and it doesn’t measure liver function, so those statements don’t fit its role.