Insulin Immunoassay Interference Due to Human Antimouse Antibodies in a Patient With Ketotic Hypoglycemia

Misinterpretation of common endocrine hormonal immunoassays can distort the clinical picture and lead to unnecessary medical workups. Potential assay inference is important to recognize when the clinical presentation and laboratory evaluation are inconsistent. This is demonstrated by the case of an 18-month-old girl who initially presented with ketotic hypoglycemia and was found on diagnostic fasting evaluation to have the triad of hypoglycemia, inappropriately high insulin levels, and low C-peptide levels—point-of-care glucose 43 mg/dL (2.39 mmol/L) (confirmatory 52 mg/dL [2.89 mmol/L]), insulin 48.1 μIU/mL (334 pmol/L), and C-peptide 0.2 ng/mL (0.07 nmol/L) concerning for factitious insulin (insulin:C-peptide ratio 4.77). On repeat diagnostic fast, insulin assays measured by liquid chromatography–mass spectrometry were incongruent with prior testing by immunoassay, demonstrating a falsely elevated insulin level when measured by immunoassay, likely due to human antimouse antibody interference (HAMA 181 ng/mL). This case represents a diagnostic challenge in which is it imperative to recognize possible immunoassay interference. It is critical to establish the difference between insulin assay interference and factitious insulin through use of alternative laboratory methods as misdiagnosis could lead to the serious implication of Munchausen by proxy resulting in the removal of a child from their home and potentially parents being charged with a crime.