OBJECTIVE: To determine the best autoantibody-based testing strategy for recruiting relatives for future intervention trials and to establish the role of islet cell antibodies (ICAs) within this strategy. RESEARCH DESIGN AND METHODS: ICAs, insulin autoantibodies (IAAs), GAD antibodies, and IA-2 antibodies were determined in serum samples at study entry of 3,655 non-diabetic first-degree relatives of patients with type 1 diabetes who were followed for a median of 5.5 years. The cumulative risk of diabetes associated with single and combined antibody marker levels of > or = 97.5th percentile in schoolchildren was calculated by using life-table analysis. RESULTS: Of the 26 relatives who developed insulin-requiring diabetes during follow-up, 16 were aged < 20 years and 7 were aged 20-39 years at study entry. Of the 23 cases aged < 40 years, 83% had IA-2 and/or GAD antibodies, and 87% had IAA and/or GAD antibodies > or = 97.5th percentile compared with 61% who had ICAs of > or = 5 Juvenile Diabetes Foundation units (JDF U). A two-step strategy with parallel testing for IA-2/GAD antibodies followed by IAA testing identified 50% of cases aged < 20 years and was associated with a 71% risk within 10 years. In subjects aged 20-39 years, this strategy conferred a 51% risk, whereas using ICAs as the second test gave 86% sensitivity and a 74% risk. Primary screening for IA-2 and/or GAD antibodies followed by testing for IAA and/or ICA antibodies achieved the highest sensitivity in both age-groups and conferred a 63% risk. In contrast, ICAs of > or = 20 JDF U (the inclusion criteria for the European Nicotinamide Diabetes Intervention Trial) gave 48% sensitivity and 35% risk. CONCLUSIONS: ICA testing can be replaced as a primary screening measure by IA-2/GAD or IAA/GAD antibody testing. The sensitivity of ICAs (used alone or in combination with IAAs) gives them a useful role in second-line testing. Combination testing could reduce the size of screening populations needed for recruitment in future intervention trials by approximately 50% compared with testing based on ICAs alone.

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