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?’