Value of the Free C-Peptide Assay

Objectives: Insulin autoimmune syndrome (IAS) is an extremely rare cause of hypoglycemia, particularly in non-Asian populations. Methods: In this report, we describe a white male patient with elevated total insulin (>100.0 µIU/mL), C-peptide, and proinsulin levels who was diagnosed with IAS due to anti-insulin antibodies. He also had a small IgG к M-protein. Results: We show that anti-insulin antibodies and/or the monoclonal protein can significantly interfere with insulin and C-peptide immunoassays and propose polyethylene glycol precipitation to quantitate free C-peptide levels as a useful assay in differentiating IAS due to anti-insulin antibodies from insulinoma. Conclusions: In patients presenting with hypoglycemi