Insulin levels can be useful predicting susceptibility to the development of type II diabetes, although C-peptide has largely supplanted insulin measurement for this role.1 The American Diabetes Association recommendations for the diagnosis of diabetes do not include the measurement of insulin levels.
Circulating antibodies to insulin, both autoantibodies and antibodies to therapeutic insulin, can interfere with the immunoassays for insulin.1,3 In the work-up of insulinoma, factitious hypoglycemia must be considered.1 C-peptide measurement is useful in this context. Since C-peptide is produced as the result of the cleavage of proinsulin in the β-cell, it is not found in pharmaceutical insulin. Thus, C-peptide levels are low with the surreptitious injection of insulin. C-peptide levels are increased when endogenous levels of insulin are elevated.