Complete laboratory diagnosis of Insulin Autoimmune Syndrome

Abstract

The definition of Insulin autoimmune syndrome includes the presence of high levels of blood insulin and insulin autoantibodies. We encountered a 45-years-old white man with a high insulin serum value that do not fit with the C-peptide result. To discard or to confirm an analytical interference and diagnose a possible Insulin Autoimmune Syndrome we performed the following investigations: dilution linearity test, heterophilic antibody blocking, polyethylene glycol precipitation, measurements with alternative assays, and gel filtration chromatography by size exclusion. The latter technique confirmed that most of the insulin was complexed with a 150-kDa protein, corresponding to immunoglobulin G, identified as insulin autoantibodies. These antibodies were responsible for hypoglycemia attacks in the patient, who had a previous autoimmune disease. This case highlights the importance of carefully analyzing the results and ruling out possible interferences, as well as considering all kinds of pathologies, even if they are infrequent.

Keywords: C-peptide; Insulin; Insulin autoantibody; Insulin autoimmune syndrome; Interference; Size exclusion chromatography.

Fig. 1

Protein standard: β-Galactosidase (460 kDa), Fab (50 kDa), Myoglobin (17 kDa), Dipeptide (0.2 kDa); and IgM mix: IgM (970 kDa), IgG (150 kDa), Albumin (70 kDa) used in the SEC procedure as reference for the determination of insulin complex.

Fig. 2

Representation of the overlay of the chromatogram of the serum patient sample and Elecsys insulin results. The high insulin concentration is found in the fraction where immunoglobulin G is expected.