Abstract
We determined the frequency of antibodies to glutamic acid decarboxylase (GAD-Ab) using a new radioimmunoassay (RIA) developed by Hoechst Japan Ltd, and examined the relatioship between GAD-Ab and islet cell antibodies (ICA). The subjects of our study were 35 patients with IDDM who were less than 25 years of age at onset (male 13, female=22;age at onset.16.46±4.9 years; duration of diabetes, 5.54±4.6years). GAD-Ab was measured by RIA using GAD purified from porcine brain as an antigen. We expressed the titer of GAD-Ab in units based on the percentage of radioactivity of the reference serum, and determined a positivity limit of 8 U.ICA was assessed with a conventional indirect immunofluorescence method. GAD-Ab was positive in 17 of 35 patients (48.6%). The frequency of GAD-Ab was 60.0%(108.8±106.0 U;mean±SD of titers) in those patients with a duration of less than 1year, 50.0%(20.2±16.5 U) with 1 to 3 years, 62.5%(58.7±111.5 U) with 3 to 5 years, 40.0%(47.3±119.1 U) with 5 to 10years, and 37.5%(26.1±33.4 U) in those with a duration exceeding 10years. The mean of titers decreased with increasing duration of diabetes. There was a relationship between ICA and GAD-Ab (p<0.05). The prevalence of ICA increased as GAD-Ab titers rose.We conclude that GAD-Ab is related to ICA, and is comparable to ICA as an IDDM-associated marker.
