Reduced endogenous insulin secretion in diabetic patients with low-titer positive antibodies against GAD

Aim: To investigate the clinical characteristics of diabetic patients with a low-titer positive for the anti-glutamic acid decarboxylase 65 antibody (GAD antibody). Methods: The subjects were 420 diabetic inpatients. The endogenous insulin secretion was estimated on the basis of the C-peptide immunoreactivity from a 24 h urine collection (uCPR). Clinical variables were compared between patients negative for the GAD antibody (GAD antibody titer < 1.5 U/mL), a low-titer positive GAD antibody (1.5 U/mL ≤ GAD antibody titer < 10 U/mL) and a high-titer positive GAD antibody (10 U/mL ≤ GAD antibody titer). Results: The low and high-titer positive GAD antibodies were found in 25 and 10 patients, respectively. The uCPR was significantly lower in both the patients with a low (37 ± 33 ug/24h) and high-titer (39 ± 27 ug/24h) positive GAD antibodies than in those negative for GAD antibodies (71 ± 52 ug/24h). The uCPR level was significantly lower in the low-titer positive GAD antibody group (29 ± 22 ug/24h) than in the negative group (67 ± 55 ug/24h) among the patients not taking insulin secretagogues. The difference disappeared in the subjects taking insulin secreagogues. In the stepwise multiple regression analysis, a low-titer positive GAD antibody was independently associated with the uCPR level. Conclusions: Endogenous insulin secretion is reduced in diabetic patients positive for GAD antibodies, even if the titer is low. Earlier initiation of insulin therapy might therefore protect the pancreatic β-cell function in diabetic patients with a low-titer positive GAD antibody.