To compare the incidence rate of IDDM in the age-groups 0–14 and 15–39 years in Antwerp, Belgium, and to compare demographic, clinical, and biological data from Antwerp IDDM patients with 92% ascertainment with those from a larger Belgian patient group with 40% ascertainment.
Incident cases of IDDM were reported by physicians of the Belgian Diabetes Registry and in Antwerp by several other sources. In Antwerp, completeness of ascertainment was calculated by the capture-recapture method. Demographic and clinical data were collected by questionnaire. Blood was sampled for HLA-DQ genotyping and, in new-inset patients, for autoantibodies.
In Antwerp, the age- and sex-standardized IDDM incidence rates were similar in both age-groups (0–14 years: 11.8/100,000; 15–39 years: 8.9/100,000). The incidence rate decreased in girls above age 15 years (6.9/100,000; P = 0.003) but not in boys (11.0/100,000). Both in Antwerp and Belgium, IDDM was diagnosed more frequently in the 15–39 years age-group (60% of all cases) than under age 15 years, with a lower prevalence of acute symptoms, ketonuria, high-risk HLA-DQ genotype, and autoantibodies against insulin, islet cells, and IA-2, but with a higher prevalence of GAD65 autoantibodies.
In Antwerp, the incidence rate of IDDM under age 15 years is intermediately high compared with the rates in other European regions. It is similar in the 15–39 years age-group, but with a marked male predominance. Demographic, clinical, and biological data show the same age-dependent heterogeneity as the data collected nationwide, with 40% ascertainment indicating the representativeness of the latter.