Reduced risk of hospital admission for childhood asthma among Scottish twins: record linkage study

A recent study of Swedish army conscripts found a reduced prevalence of asthma and allergic rhinitis among twins.1 We analysed routine data on hospital admissions in Scotland to compare risks of asthma and other respiratory complaints among twins and singletons. We identified all twins born in Scotland during 1981–4 from computerised maternity records. Subsequent admissions of twins to Scottish hospitals during 1981–94 were ascertained by probability matching on the basis of date of birth, sex, and surname. This matching is considered 99% accurate for singletons, but for twins it is reliable only at the level of the pair: which twin is admitted cannot be identified with certainty. We identified hospital admissions for respiratory disease (ICD-9 (international classification of diseases, 9th revision) codes 464, 466, 480-486, and 490-496) for all Scottish children born during …