Which test is used to differentiate between inappropriate exogenous insulin administration and endogenous insulin production?

The C-Peptide test is utilized to differentiate between inappropriate exogenous insulin administration and endogenous insulin production effectively. C-Peptide is a byproduct of insulin production that is released in equimolar amounts with insulin from the pancreas. When endogenous insulin is produced, both insulin and C-Peptide levels will rise; however, if insulin is externally administered, C-Peptide levels remain low or undetectable because the exogenous insulin does not trigger C-Peptide release from the pancreas. This allows healthcare providers to determine whether the insulin present in the patient’s system is naturally produced or has been administered externally.

In contrast, the insulin tolerance test assesses the body's sensitivity to insulin rather than the source of insulin itself. Fasting insulin levels provide a snapshot of the insulin present in the bloodstream but do not indicate whether it is endogenous or exogenous. The oral glucose tolerance test is designed to assess how well the body handles glucose rather than identifying the source of insulin. Hence, the C-Peptide test remains the most relevant for distinguishing between these two conditions.