Antibodies to GAD and glycemic control in recent-onset IDDM.

Objective: To analyze the effect of antibodies to glutamic acid decarboxylase (GAD65Ab) and islet cells (ICA512Ab) on glycemic control early in IDDM.
Research Design and Methods: GAD65Ab and ICA512Ab were measured twice in 35 patients (10 male, 25 female; age 10-40 years) initially within 2 years of diagnosis and again 1 year later. The glycosylated hemoglobin was measured one to four times each year, and the average glycosylated hemoglobin for the preceding year was calculated each time the antibodies were measured.
Results: The mean HbA1 at the time of the initial evaluation was 8.04 +/- 0.30 (reference range 4.7-7.3% for nondiabetic patients), the average GAD65Ab index was 0.735 +/- 0.306, and the mean ICA512Ab index was 1.94 +/- 0.65. The GAD65Ab index correlated with HbA1 (r = 0.41, P < 0.025), whereas the ICA512Ab index did not (r = 0.13). One year later, the mean GAD65Ab index was 0.94 +/- 0.34, the mean ICA512Ab index was 1.04 +/- 0.40, and the mean HbA1 was 9.03 +/- 0.30. The GAD65Ab index correlated with HbA1 (r = 0.61 P < 0.001), whereas the ICA512Ab index did not (r = -0.06). Stratification of patients into tertiles according to the average GAD65 index revealed, at the follow-up evaluation, that the better glycemic control in the lowest GAD65Ab tertile was accomplished with significantly less insulin (0.43 +/- 0.08 U/kg for the lowest tertile vs. 0.71 +/- 0.09 and 0.64 +/- 0.09 for the middle and highest tertiles, respectively; P < 0.05).
Conclusions: In summary, patients with IDDM and low GAD65Ab have better glycemic control even though they require less insulin. The ICA512Ab index, however, fails to correlate with glycemia.