Insulin immunoassays in the detection of insulin analogues in factitious hypoglycemia.

OBJECTIVE: To report a case of surreptitious insulin use and to review the differences in insulin assays and how they may be optimally used in testing for factitious hypoglycemia.
METHODS: We describe the clinical, physical, and laboratory findings of the study patient and review the current relevant literature regarding surreptitious insulin use and detection by insulin assays.
RESULTS: A 36-year-old man with type 1 diabetes mellitus was admitted to the hospital with a serum glucose concentration of 28 mg/dL. He reported taking very small amounts of glargine and aspart insulin (16 units per day). Mitigating endocrine causes of hypoglycemia (eg, adrenal insufficiency) were investigated and excluded. His serum insulin concentration was 14 microIU/mL (reference range, 3-25 microIU/mL) despite his statement that he was not taking human insulin. Serum insulin concentration steadily decreased over the hospital course. The declining insulin levels, coupled with resolution of hypoglycemia, suggested the abuse of human insulin. After confrontation with this evidence, he was referred to psychiatric services. Evaluation of surreptitious human insulin use is straightforward since it can be distinguished from endogenous insulin excess (eg, insulinoma) by existing assays for insulin, C-peptide, and proinsulin. However, the increasing use of insulin analogues in lieu of human insulin has made such evaluations more difficult because commercially available insulin assays detect these synthetic insulins with varying sensitivity.
CONCLUSIONS: Factitious insulin use remains an uncommon yet hazardous problem. Human insulin is readily available without a prescription, providing easy access to an extremely dangerous substance. Understanding the specific detection abilities of each assay is essential to the evaluation of factitious insulin-induced hypoglycemia.