Discrepancy of glutamic acid decarboxylase 65 autoantibody results between RSR radioimmunoassay and enzyme‐linked immunosorbent assay in patients with type 1 diabetes is related to autoantibody affinity

AIM/INTRODUCTION: Autoantibodies to the 65 kDa isoform of glutamic acid decarboxylase (GADA) are a valuable diagnostic and predictive marker for type 1 diabetes. Recently, it has been reported that a significant proportion of sera in the commercial RSR radioimmunoassay (RIA) that have tested positive for GADA have then turned negative in RSR enzyme‐linked immunosorbent assay (ELISA) tests in patients with type 1 diabetes. The present study aimed to investigate whether the GADA result discrepancies between RSR‐RIA and RSR‐ELISA are related to autoantibody affinity. METHODS: GADA affinity was measured by a competitive binding experiment using unlabeled recombinant human GAD65 in 12 discordant samples (5 RIA[+]/ELISA[−] and 7 RIA[−]/ELISA[+] sera). Furthermore, the effect of the initial incubation time on the GADA positivity was also examined using the ELISA test. RESULTS: GADA affinities were >10(10) L/mol in two of five RIA(+)/ELISA(−) and all of seven RIA(−)/ELISA(+) sera. After an initial incubation time longer than the recommended 1 h, the GADA titer in three of five RIA(+)/ELISA(−) sera and all RIA(−)/ELISA(+) sera increased 1.6‐ to 100‐fold. However, the titer in 12 GADA‐negative sera from healthy controls remained unchanged after the longer incubation. The increment ratio of GADA titer was positively correlated with GADA affinity (r = 0.991, P < 0.001). CONCLUSIONS: The RSR‐RIA test identifies both high‐ and low‐affinity GADA, whereas the RSR‐ELISA test identifies only high‐affinity GADA. A longer initial incubation time in the RSR‐ELISA test increases the sensitivity of GADA with the same specificity in patients with type 1 diabetes.