When would C-peptide testing be most informative?

• A. To distinguish type 1 (low/absent C-peptide) from type 2 (preserved or high C-peptide) and to assess endogenous insulin production
• B. To monitor blood glucose in real time
• C. To evaluate pancreatic glucagon release
• D. To decide need for SGLT2 inhibitors

Answer: (A)

C-peptide testing is most informative because it reflects how much insulin the pancreas is still making. Since C-peptide is released in equal amounts with insulin but is not cleared by the liver and has a longer half-life, it serves as a reliable marker of endogenous insulin secretion. This makes it useful to tell apart autoimmune-type diabetes, where the beta cells are largely destroyed and C-peptide is low or absent, from insulin-resistant-type diabetes, where C-peptide is preserved or even high due to ongoing insulin production. It’s especially helpful when the patient’s type is uncertain or when evaluating residual beta-cell function in someone who may be using exogenous insulin, to guide treatment decisions.

It's not used for real-time glucose monitoring, which relies on glucose meters or continuous glucose monitoring devices. It doesn’t assess glucagon release from alpha cells, and it isn’t a factor in deciding whether to use SGLT2 inhibitors, which depend more on overall glycemic control, kidney function, and cardiovascular considerations.