C-peptide testing is indicated in the evaluation of diabetes for which purpose?

• A. Distinguishing type 1 from type 2 and assessing residual beta-cell function, including evaluation of suspected monogenic diabetes
• B. Measuring HbA1c
• C. Detecting retinopathy
• D. Predicting insulin doses

Answer: (A)

C-peptide testing reflects how much insulin the body is still producing, because C-peptide is released in equimolar amounts with endogenous insulin when proinsulin is split. This makes it a useful marker to distinguish autoimmune destruction of beta cells (type 1) from insulin resistance with preserved or residual beta-cell function (type 2) and to identify cases of suspected monogenic diabetes, where some beta-cell activity remains and autoantibodies may be absent.

In practice, a low or undetectable C-peptide suggests little to no endogenous insulin production, supporting a type 1-like picture, while a normal or elevated C-peptide indicates ongoing insulin production, consistent with type 2 or monogenic forms. HbA1c measurement, retinopathy screening, and direct insulin-dosing guidance rely on separate assessments, so C-peptide specifically informs the diagnosis and the level of residual beta-cell function rather than those other aspects.