Antibody to the M(r) 65,000 isoform of glutamic acid decarboxylase are detected in non-insulin-dependent diabetes in Japanese

It was recently reported that antibodies against glutamic acid decarboxylase (GADAb) have a high positive predictive value for insulin-dependency in non-insulin-dependent diabetic (NIDDM) patients. We studied 289 patients classified at onset as having NIDDM. Patients positive for GAD65Ab had a disease onset at a younger age, lower body mass index (BMI) and lower serum C-peptide concentration, and were more often treated with insulin. Among 73 insulin-treated patients, patients with lower C-peptide level (n = 30, C-peptide < or = 0.9 ng/ml) showed a higher frequency of GAD65Ab (46.7%) than patients with normal C-peptide (n = 53, C-peptide > 0.9; 7.5%, P < 0.0001). The 206 remaining non-insulin-treated patients were divided into 48 short-duration (less than 5 years from diagnosis) and 158 long-duration patients. Frequency of GAD65Ab in short-duration patients (10.4%) was significantly higher than that in long-duration patients (3.2%, P < 0.05). Among short-duration patients, there was no significant difference in C-peptide levels between GAD65-positive and negative patients (2.175 and 2.226 ng/ml). In conclusion, GAD65Ab, a marker of insulin deficiency, may predict the development of insulin dependency in non-insulin-dependent Japanese diabetic patients before serum C-peptide concentration decreases.