Study design and study sample
The Swedish Twin Study on Prediction and Prevention of Asthma (STOPPA)
is a population-based, cross-sectional twin study on childhood asthma in
discordant, concordant, and healthy concordant twins22. The study participants were recruited from the
Child and Adolescent Twin Study in Sweden (CATSS) 23,
a study initiated in 2004 that included all twins born from July 1992
and onwards, identified through the Swedish Twin Registry24. Validated questions on asthma ever (yes/no) and
wheezing (current or after three years of age) were used to identify 9-
to 14-year-old twins discordant and concordant for asthma or wheeze and
healthy control twins according to the International Study of Asthma and
Allergies in Childhood (ISAAC) 25. Monozygotic (MZ)
and same-sex dizygotic (DZ) twin pairs who were raised together were
recruited to STOPPA from all parts of Sweden and distributed throughout
the whole year. The study population (n = 752) was classified as asthma
concordant (31%), asthma discordant (38%), and healthy concordant
(31%), according to the recruitment algorithm of asthma status, as
described in detail elsewhere 22. One twin pair had
unknown zygosity, and 138 individuals had missing data on inhaled
corticosteroid steroid (ICS) use. The final analytic sample consisted of
612 individuals (81.4% of the study population with full information on
all covariates). The study was approved by the Regional Ethical review
board in Stockholm, Sweden. Informed consent was obtained from all the
participants and their parents.