GAD Antibody Positivity Predicts Type 2 Diabetes in an Adult Population

OBJECTIVE: To evaluate the significance of GAD antibodies (GADAs) and family history for type 1 diabetes (FH(T1)) or type 2 diabetes (FH(T2)) in nondiabetic subjects. RESEARCH DESIGN AND METHODS: GADAs were analyzed in 4,976 nondiabetic relatives of type 2 diabetic patients or control subjects from Finland. Altogether, 289 (5.9%) were GADA(+)—a total of 253 GADA(+) and 2,511 GADA(−) subjects participated in repeated oral glucose tolerance tests during a median time of 8.1 years. The risk of progression to diabetes was assessed using Cox regression analysis. RESULTS: Subjects within the highest quartile of GADA(+) (GADA(+)(high)) had more often first-degree FH(T1) (29.2 vs. 7.9%, P < 0.00001) and GADA(+) type 2 diabetic (21.3 vs. 13.7%, P = 0.002) or nondiabetic (26.4 vs. 13.3%, P = 0.010) relatives than GADA(−) subjects. During the follow-up, the GADA(+) subjects developed diabetes significantly more often than the GADA(−) subjects (36/253 [14.2%] vs. 134/2,511 [5.3%], P < 0.00001). GADA(+)(high) conferred a 4.9-fold increased risk of diabetes (95% CI 2.8–8.5) compared with GADA(−)—seroconversion to positive during the follow-up was associated with 6.5-fold (2.8–15.2) and first-degree FH(T1) with 2.2-fold (1.2–4.1) risk of diabetes. Only three subjects developed type 1 diabetes, and others had a non–insulin-dependent phenotype 1 year after diagnosis. GADA(+) and GADA(−) subjects did not clinically differ at baseline, but they were leaner and less insulin resistant after the diagnosis of diabetes. CONCLUSIONS: GADA positivity clusters in families with type 1 diabetes or latent autoimmune diabetes in adults. GADA positivity predicts diabetes independently of family history of diabetes, and this risk was further increased with high GADA concentrations.