Seasonality of type 1 diabetes mellitus in children and its modification by weekends and holidays: retrospective observational study.

BACKGROUND Diagnoses of type 1 insulin dependent diabetes mellitus are generally more common in winter, although this seasonal pattern has not been observed in children of preschool age (0-4 years) or in all countries. AIMS To confirm the persistence of seasonality and the influence of age, holidays, and weekends. METHODS We extracted data on date of birth, date of presentation, age, and sex of children diagnosed with diabetes and registered with the Scottish Study Group for the Care of Diabetes in the Young. Cosinor analysis was applied to monthly and mid-monthly data. Two sample Z tests were used to compare the epochs 1984-1992 and 1993-2001. RESULTS Some 4517 children between 0 and 14 years of age (2407 male and 2110 female) presented with IDDM between 1 January 1984 and 31 December 2001. Seasonality was evident in children above 4 years of age with amplitudes of 19.5-25.7% and peaks between mid December and mid January. Presentation was strongly influenced by weekends and holiday periods, with reduced presentations in December compared with November and January, and with the lowest presentations in July (the main Scottish holiday month). Using mid-month to mid-month data did not change the overall seasonality but did improve the fits for cosinor analysis. Mondays and Fridays were the most common days for presentation. CONCLUSION Initial presentation of IDDM in Scotland follows a stable seasonal pattern in all but the youngest children with lower rates of presentation in holiday periods and at weekends for all age groups.