Increased C-Peptide Immunoreactivity in Insulin Autoimmune Syndrome (Hirata Disease) Due to High Molecular Weight Proinsulin

Abstract

Background Determination of C-peptide is important in the investigation of unexplained hyperinsulinemic hypoglycemia because a high C-peptide concentration usually indicates endogenous insulin hypersecretion. Insulin autoimmune syndrome (IAS) denotes hyperinsulinemic hypoglycemia due to insulin-binding antibodies that prolong insulin half-life. C-peptide clearance is considered to be unaffected, and although a marked C-peptide immunoreactivity in hypoglycemic samples has been reported, it has been suspected to be artifactual. High-resolution mass spectrometry enables examination of the basis of C-peptide-immunoreactivity in IAS. Methods Precipitation of plasma with polyethylene glycol was followed by C-peptide immunoassay. Plasma peptides extracted by solvent precipitation were characterized by nano-LC–MS/MS and analyzed using an untargeted data-dependent method. Peptides related to proinsulin, in amino acid sequence, were identified using proprietary bioinformatics software and confirmed by repeat LC–MS/MS analysis. Gel filtration chromatography coupled to LC–MS/MS was used to identify proinsulin-related peptides present in IAS immunocomplexes. Results were compared with those from C-peptide immunoassay. Results Polyethylene glycol precipitation of IAS plasma, but not control plasma, depleted C-peptide immunoreactivity consistent with immunoglobulin-bound C-peptide immunoreactivity. LC–MS/MS detected proinsulin and des 31,32 proinsulin at higher abundance in IAS plasma compared with control plasma. Analysis by gel filtration chromatography coupled to LC–MS/MS demonstrated proinsulin and des 31,32 proinsulin, but no C-peptide, in plasma immunocomplexes. Conclusions Antibody binding can enrich proinsulin and des 31,32 proinsulin in IAS immunocomplexes. Proinsulin cross-reactivity in some C-peptide immunoassays can lead to artifactually increased C-peptide results.

Insulin/C-peptide molar ratio is not influenced by plasma glucose level in patients with EIAS

“…Prior studies have demonstrated that immunological assays, such as ELISA, can lead to an increased level of insulin and C-peptide due to the inability to detect proinsulin [18]. Mass spectrometry is not affected by the antigen-antibody complex, and a more accurate C-peptide can be determined [18,19]. Therefore, our conclusions should also be veri ed after detecting insulin and Cpeptide levels by mass spectrometry.…”

Secondary Hirata Syndrome in a Neonate: Practical Approach and Management

“…The removal of C-peptide remains unchanged, making the ratio greater than 1 ( 12-14 ). Furthermore, in IAS, C-peptide levels may be raised as a consequence of the interference with the available immunoassays; proinsulin cross-reactivity in some C-peptide immunoassays can lead to falsely increased C-peptide results ( 15 ). IgG antibodies are eliminated from the circulating blood of the baby in 3 to 6 months, so the hypoglycemia episodes could appear in that time window.…”

Resolution of dysglycaemia after treatment of monoclonal gammopathy of endocrine significance

“… 1 Detectable C-peptide immunoreactivity concurrent in a hypoglycaemic sample was of high molecular weight and could represent antibody-bound proinsulin cross-reactivity in immunoassay. 1 , 26 …”