Variables
Asthma. All twins and their parents completed questionnaires. The
parental questionnaire, which has previously been validated against
health care registers with good agreement26, included
questions on the parent’s lifestyle, background, and medical history,
followed by a section on each twin’s general health status, lifestyle,
and medical history. Everyone that replied positively to the ISAAC
validated parental question ’Does he/she have, or has he/she had
asthma?’ 23,25 was then directed to other
asthma-related questions and ‘reported current asthma’ was then defined
as reporting positively to the question ’Does he/she still have
asthma?’ 22.
FENO. Exhaled nitric oxide was measured during an
exhalation of at least 6 seconds at a flow of 50 ml/s
(FENO), measured with a hand-held electrochemical
analyzer (NIOX Mino, Aerocrine, Solna, Sweden) according to the
guidelines 27. The average integer value of
FENO (parts per billion) was recorded based on two
consecutive measurements if they differed by less than 5% or based on
three measurements if they differed by >5%.
Sensitization to airborne allergens. More than 90% of the
participants underwent blood sampling, and serum was analysed for IgE
antibodies to Phadiatop® 28, Thermo Fisher Scientific,
Uppsala, Sweden, a screening test for sensitization to a mix of common
inhalant allergens (birch, timothy, mugwort, cat, dog, horse, house dust
mites [Dermatophagoides pteronyssinus and farina ], and
mold [Cladosporium herbarum ]). Sensitization was regarded as
a level of ≥0.35 kUA/L corresponding to a fluorescence
intensity of 168 response units and this defined the categorical (1/0)IgE positive variable. The numeric IgE level to Phadiatop®, here
termed the allergen-specific IgE level , was used as a
continuous IgE variable and has been described previously29. IgE values below the level of quantification of
0.1 kUA/L were assigned a value 0.09
kUA/L, and values above 100 kUA/L were
set at 100 kUA/L, as described elsewhere30. All samples were analysed at the Department of
Clinical Immunology and Transfusion Medicine at the Karolinska
University Hospital Solna, Sweden.
Eosinophils. Samples of venous blood were collected, and the
numbers of blood eosinophils (1 × 109 counts/L) were
counted at the local cites of Clinical Chemistry Laboratory.
Inhaled corticosteroids . Parents confirmed inhaled corticosteroid
(ICS) treatment by answering yes to the questions ’Does he/she
have, or has he/she had asthma?’ and ‘Has your child used ICS
treatment regularly during the last 12 months?’
Zygosity. Data on zygosity were retrieved from the CATSS study. A
majority of the twins had their zygosity determined by DNA analysis
(84.3%), with the remaining assessed via an algorithm of five questions
on twin similarity, a validated technique to determine zygosity with at
least 95% accuracy 23.
Age. Information on age was collected from the questionnaires and
is included as a covariate in the analyses.
Socioeconomic status (SES). As a proxy for SES, we used the
parental (maternal or paternal) highest education retrieved from the
questionnaire.
Any parental asthma. To assess the parental history of asthma, we
collected the following item from the questionnaires: ‘Does the
mother/father have asthma?’ and created a new variable ‘any parental
asthma,’ based on whether either the mother or father or both had
asthma.
Parental current smoking. Smoking was assessed from the
questionnaire with the following question: ‘Does the mum/dad
smoke?’