When discordant insulin and C-peptide levels lead to a medical diagnosis in a patient with transient hypoglycemia: Varying degrees of interference of insulin-antibody complexes on three insulin immunoassays.

Background: Determining the cause of hypoglycemia partly relies on blood insulin and C-peptide assays. Although the pancreatic secretion of these peptides is equimolar, discrepancies in their concentrations may occur. Case presentation: We report the case of a 73-year-old woman with type 2 diabetes mellitus (T2DM) and a history of gastric bypass. The T2DM was initially treated with insulin analogs, which were interrupted due to transient hypoglycemia episodes three years before hospitalization in our endocrinology department. During this hospitalization, the most common etiologies of hypoglycemia were excluded. Fasting insulin level was high (190 mIU/L, reference values (RV): 5-25) on Architect i2000 (an assay recognizing insulin analogs) despite normal blood C-peptide (4.5 µg/L, RV: 0.8-5.2) and slight hypoglycemia (4.5 mmol/L, RV: 4.6-6.1). Insulin level using the Elecsys assay (an assay with low sensitivity to insulin analogs) was very high (>1000 mIU/L, RV: 2.6-24.9). This pattern was observed on several samples, including some taken during a fasting test. Insulin level was only slightly increased using the Mercodia iso-insulin ELISA kit (an assay recognizing insulin analogs). These results excluded an exogenous insulin intake and were suggestive of an interference on insulin assays. To explore the latter possibility, free anti-insulin antibodies were measured and found strongly positive. The presence of interfering insulin-antibody complexes was further investigated using gel filtration chromatography, polyethylene glycol precipitation, and dilution test. Based on these findings, an insulin autoimmune syndrome (IAS) was suspected to cause the hypoglycemic episodes observed. Conclusion: Although a discrepancy between blood insulin and C-peptide levels points to insulin analog intake, IAS should also be considered, particularly in a patient with transient hypoglycemia. IAS is characterized by the presence of insulin-antibody complexes, which can induce varying degrees of interference on insulin immunoassays and may lead to discordant insulin and C-peptide levels according to the insulin immunoassay used.