Evaluation of precision, comparability and linearity of MAGLUMI™ 2000 Plus GAD65 antibody assay

Chiara Cosma, Andrea Padoan, Mario Plebani


Background: Type 1a diabetes (TD1a) is an autoimmune chronic disease and more than 85% of newly diagnosed patients presented elevated levels of glutamate decarboxylase 65-kDa isoform (GAD65) antibodies in serum. Aim of this study was to evaluate the MAGLUMI™ 2000 Plus GAD65 antibody chemiluminescence immunoassay (CLIA) precision, linearity and comparability with EUROIMMUN Anti-GAD enzyme-linked immunosorbent assay (ELISA).
Methods: MAGLUMI™ 2000 Plus (Snibe, Keyuan Road, Nanshan District, Shenzhen, China) precision was estimated adopting the Clinical and Laboratory Standards Institute (CLSI) EP15-A3 protocol and compared against the manufacturer’s claimed values. A series of 135 specimens were used for comparing GAD65 antibodies between MAGLUMI™ 2000 Plus and EUROIMMUN Anti-GAD ELISA (EUROIMMUN Medizinische Labordiagnostika AG, Lubeck, Germany). Linearity of dilution was assessed by mixtures of high-level pools and a low-level pool of samples.
Results: MAGLUMI™ 2000 Plus repeatability and intermediate precision were measured at 22.2, 58.4 and 101.2 IU/mL levels of GAD65 antibody. At those levels, results of repeatability were 2.96%, 1.02% and 1.95%, and of intermediate imprecisions were 4.62%, 3.34% and 2.17%. Comparability showed that a detectable proportional bias was present between the two assays, being GAD65 antibody levels underestimated by MAGLUMI™ 2000 Plus CLIA assay with respect to EUROIMMUN Anti-GAD ELISA. However, using the cut-off suggested by the manufacturer for dichotomizing results, a good agreement was found between the two methods (90.4%). Linearity was optimal within the range 60–100 IU/mL, poor in the lower range.
Conclusions: MAGLUMI™ 2000 Plus GAD65 antibody CLIA assay showed a satisfactory imprecision, supporting the utilization of this assay in clinical practice for diagnosing TD1a. However, our data confirm the need for better standardization/harmonization between GAD65 antibody assays.