A Simple Solid-Phase Radioimmunoassay for Glutamic Acid Decarboxylase (GAD) Antibodies in Patients with Diabetes Mellitus

Abstract

Antibodies to glutamic acid decarboxylase (GAD Ab's) provide an important serologic marker for insulin-dependent diabetes mellitus (IDDM), but the diagnostic sensitivity and specificity of GAD antibodies have yet to be defined in assay systems suitable for screening a large number of samples. Therefore, using affinity purified GAD from rat brain we have developed a simple and sensitive solid-phase radioimmunoassay for measuring antibodies against the isoforms GAD₆₅ and GAD₆₇ and have examined a large number of serum samples from Japanese patients with IDDM of short-duration (less than 3 years) as well as from those with non-insulin-dependent diabetes mellitus (NIDDM). We detected GAD Ab in 62% (37 of 60) of the patients with IDDM, but only in 8% (4 of 51) of those with NIDDM. Almost all the patients with other autoimmune diseases and the healthy controls had no GAD Ab at all. Seventy-eight percent of the IDDM patients with GAD Ab concomitantly had islet cell cytoplasmic antibody (ICA), and there was a significant correlation between both autoantibody levels. This assay provides for rapid and simple screening for the early diagnosis of IDDM.