Clinical relevance of the Maglumi immunoassay for anti-GAD65 in diabetes and neurological disorders: Analytical challenges and diagnostic insights

Introduction

Anti-GAD65 is an established diagnostic biomarker for autoimmune diabetes and neurological syndromes associated with GAD antibody-spectrum disorders. Automated chemiluminescence-based immunoassays such as Maglumi are increasingly used, but their behavior at high antibody concentrations remains poorly characterized. We report the analytical concordance between Maglumi and radioimmunoassay (RIA), the cut-off titer that differentiates neurological from diabetic patients, and the hook effect of the immunoassay.

Materials and methods

Anti-GAD65 levels were analyzed in samples from 90 patients with suspected autoimmune diabetes or neurological syndromes, using Maglumi 2000® and RIA (Medipan®). Analytical concordance was assessed by Passing–Bablok regression, Bland–Altman plots, and Cohen’s kappa. ROC curve analysis identified diagnostic thresholds, and dilution studies were performed in samples with unexpectedly low values to assess a potential hook effect.

Results

Maglumi results were approximately five times higher than RIA results (slope = 5.364; R² = 0.908); agreement was higher in endocrine (R² = 0.998) than in neurological patients (R² = 0.891). A cutoff of 6778 IU/mL discriminated neurological patients from diabetic patients (AUC = 0.96). Sixteen samples showed a hook effect, with direct (undiluted) values ≤273 IU/mL, while the corrected titers after dilution exceeded 20,000 IU/mL. A linear regression confirmed an inverse relationship between direct and true titers (r = –0.79, p < 0.05).

Discussion

Maglumi anti-GAD65 immunoassay provides reliable discrimination between endocrine and neurological patients but requires method-specific cutoffs to avoid misclassification. Hook effect was frequent in high-titer neurological samples, potentially leading to misdiagnosis, underscoring the need for routine dilution protocols to improve diagnostic reliability.

Conclusions

Standardized dilution protocols and appropriate cutoffs are essential to ensure reliable anti-GAD65 testing with Maglumi chemiluminescent immunoassay, particularly in neurological indications.