LONG-TERM RISK OF IDDM IN FIRST-DEGREE RELATIVES OF PATIENTS WITH IDDM

Due to a short observation period previous studies may have underestim ated prevalence and recurrence risk of IDDM in relatives of IDDM patie nts. To obtain a more exact life-time risk estimate we identified 310 probands, representative of Danish IDDM patients, characterized by cur rent age more than 50 years, age at onset 40 years or less and diabete s duration of more than 30 years. Family data were obtained from 291 p robands. Mean ''observation'' times (age) (+/-SD) for siblings (n = 55 3) and offspring (n = 359) were 59.4 +/- 16.1 years and 33.8 +/- 8.8 y ears, respectively. Of the probands 73 (25.1 %) had at least one first -degree relative with IDDM. Seventeen percent had at least one affecte d sibling. An increase from 10.4 % to 22.4 % of having first-degree re latives with IDDM among probands with age at onset below 20 years was observed during the period from proband at age 21 years up to 1 Septem ber 1992. Among affected siblings 48 % of the second cases were affect ed more than 10 years after the first affected sibling. Using the life -table method cumulative recurrence risks from time of birth were calc ulated for siblings up to age 30 years of 6.4 % and up to age 60 years of 9.6 %. For offspring the risk up to age 34 years was 6.3 %. In add ition, we present a life-table method evaluating the cumulative recurr ence risk from time of onset in the proband, as this is the most relev ant when giving genetic counselling. In conclusion, the long-term risk s of IDDM in siblings and offspring are high compared to that shown in previous reports.