INTRODUCTION
As many as 70-80% of children with asthma develop their first symptoms before the age of 51. An early diagnosis of asthma is crucial for several reasons: the increase in asthma incidence in recent decades has been principally explained by rising asthma rates in children <3 years old2; significant declines in lung function in may occur during the preschool years; and overall asthma morbidity (activity limitation, sleep disturbance, urgent care and emergency department [ED] visits, and hospitalizations) is higher in young children than in older children and adolescents3.
Several wheezing phenotypes coexist at the preschool age. Therefore, despite the importance of an early diagnosis, identifying which preschoolers with recurrent wheeze have asthma –as opposed to other, transient causes of wheezing– remains a significant challenge. Several factors make the diagnosis of asthma difficult in this age group, including the fact that parents frequently report wheezing as a catch-all term for respiratory sounds or noisy breathing. Direct observation by a physician and assessment of bronchodilator response is very useful, but objective measures of lung function and reversible obstruction are difficult in young children.
There are several models in preschoolers that attempt to predict an asthma at school and adolescent ages4. Among these, the asthma predictive index (API)5 is widely utilized because it is simple, inexpensive, minimally invasive, and it has been validated in several independent populations4. The API has a compelling positive likelihood ratio (LR ~7.4 when applied by 3 years of age, for predicting a diagnosis of asthma by age 6), making it helpful in the identification of children at high risk for asthma; but its negative LR (~0.75) is less helpful in ruling out the risk of incident asthma4. A recent cross-sectional study in preschoolers reported that the questionnaire-based diagnosis of asthma is associated with a positive API6. Our hypothesis is that the API could be used as a surrogate for asthma diagnosis in preschoolers from a longitudinal birth cohort.