title: Age-and sex-specific seasonal variation of venous thromboembolism in patients with and without family history: a nationwide family study in Sweden.
abstract: Seasonal variation in venous thromboembolism (VTE) risk in individuals with familial predisposition to VTE has not been explored. This nationwide study aimed to determine whether there are age- and sex-specific seasonal differences in risk of hospitalisation of VTE among individuals with and without a family history of VTE. The Swedish Multi-Generation Register was linked to Hospital Discharge Register data for the period 1964-2010. Seasonal variation in first VTE events in 1987-2010 for individuals with and without a family history of VTE (siblings or parents) was determined by several independent methods. Stratified analyses were performed according to age, sex, and VTE subtype (pulmonary embolism [PE] or deep venous thrombosis [DVT]). Seasonal variation in VTE incidence, mostly with a peak during the winter, was observed in both sexes in individuals with and without family history with overall peak-to-low ratios (PLRs) of 1.15 and 1.21, respectively. The peak day was December 25 and February 1 for those with and without a family history of VTE, respectively. Seasonal variation was strongest among individuals aged >50 years. Among individuals aged 0-25 years with a family history, the peak for VTE was in July (PLR = 1.20). Significant seasonal variation was observed for PE and DVT with the exception of DVT among those with a family history (PLR = 1.01). In conclusion, our data support the presence of a modest seasonal variation of VTE among individuals with and without a family history of VTE. However, young age and family history may modify and attenuate the effect of season on VTE.
authors.affiliation: Dr. Bengt Zöller, Center for Primary Health Care Research, CRC, Building 28, Floor 11, Jan Waldenströms gata 35, Skåne University Hospital, S-205 02 Malmö, Sweden, Tel.: +46 70 6691476, Fax: +46 40 391370, E-mail: bengt.zoller@med.lu.se 
authors.forename: Bengt
authors.initials: B
authors.lastname: Zöller
authors.forename: Xinjun
authors.initials: X
authors.lastname: Li
authors.forename: Henrik
authors.initials: H
authors.lastname: Ohlsson
authors.forename: Jan
authors.initials: J
authors.lastname: Sundquist
authors.forename: Kristina
authors.initials: K
authors.lastname: Sundquist
mesh_terms: D000293:Adolescent; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D018450:Disease Progression; D005190:Family; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D008875:Middle Aged; D010375:Pedigree; D012306:Risk; D012621:Seasons; D012737:Sex Factors; D013548:Sweden; D054556:Venous Thromboembolism / Q000453:epidemiology* / Q000235:genetics; D055815:Young Adult
keywords: Venous thrombosis; epidemiology; familial thrombosis; risk factors; seasons