Wheezing is common in preschool children and affects quality of life. Although asthma treatments such as inhaled steroids (ICS), which reduce swelling (inflammation) in the airways are used in this age group, they are often ineffective. That is because only some preschool children have the type of inflammation (known as Type 2 inflammation) that responds to ICS, thus many children are being unnecessarily exposed to side effects. It is difficult diagnosing Type 2 inflammation through history and examination, thus other indicators are needed to ensure ICS are only given to children who will benefit. These indicators are commonly known as biomarkers, and we are trying to find out if they are useful. We want to measure three biomarkers, without changing children's treatment. The first is blood eosinophils. which can be measured using a finger prick sample (like the blood drop used for measuring sugar levels in diabetic children). The second is to determine if allergic sensitization is present to allergens that are breathed in; these will be house dust mite, grass pollen, tree pollen, cat and dog hair. The final biomarker is a molecule that is produced in the airways of preschool children with Type 2 inflammation, called nitric oxide (NO). This is easily obtained, by having children breathe through a mask and collecting their breath in a bag, measuring NO later on. The children will be followed up with monthly electronic questionnaires and 3-monthly visits (virtual or face-to-face) for a year to evaluate whether these markers individually or in combination relate to subsequent wheezing outcomes, and how acceptable the measurements are to families using a questionnaire and focus group approach. The results will form the basis of the design of a national trial of biomarker-driven therapy in such children.

Inclusion Criteria:

1. Patients aged one to five years old presenting to primary care or emergency department or urgent care centre or identified from primary care records and have been diagnosed with wheezing by their GP or paediatrician who has decided to prescribe any bronchodilator, ICS or montelukast on clinical grounds
2. Parents/Carers able to understand and familiarize themselves with the study and are willing to provide informed consent

Exclusion Criteria:

1. Inability to understand and cooperate with study procedures
2. Significant co-morbidity (respiratory or otherwise), for example cystic fibrosis (excluding atopic disorders such as eczema, allergic rhinitis and food allergy)
3. Withholding or withdrawal of informed consent
4. Severe procedural anxiety (needle phobia)
5. Child is already enrolled in another study involving investigational medicinal product (CTIMP)
6. History of anaphylaxis or near-fatal asthma that resulted in intubation / assisted ventilation.

Children aged one to five years old presenting either to primary care with wheezing episodes or to emergency departments or urgent care centres with an acute attack of wheeze or children identified from primary care records and have been diagnosed with wheezing by their GP or paediatrician.

Data on children's age, gender and ethnicity were provided by their parents. The baseline analyses apply only to preschool children aged 1-5 years old (not their parents).

Only during biomarker testing (i.e., at single baseline visit and at second visit 3 months post-baseline)

Adverse events were recorded for the biomarker measurements but not during the observation period.

FeNO measurements were not obtained in all children due to delayed appreciation that the gas collection bag was faulty. However, a mitigation strategy was utilised to increase the number of children who had FeNO recorded. Children with FeNO measurement at baseline and those with a FeNO measurement in the second, but not baseline testing were combined, giving 68 children with FeNO measurements.