Background Asthma is one of the most common chronic diseases among children, adolescents and young adults. The use of drugs to treat asthma has not been widely investigated in the Norwegian population and there has been a lack of longitudinal data. Most studies have been conducted in confined age groups and geographic regions. The occurrence of additional health problems in the young population with asthma has important implications for the management of asthma and needs investigation. Asthma is a clinical diagnosis that is not easily captured in population-based studies and there is no agreement on a gold standard for measuring asthma in epidemiology. Prescription data on asthma drug use may be a useful proxy measure for identifying individuals with current asthma in the population.
Objectives The main objectives of this thesis were to study issues related to asthma in the Norwegian population of children, adolescents and young adults. Three areas have been studied: Asthma drug use, asthma drug use as a proxy measure of asthma, and selected additional diseases and drug treatments occurring in individuals with asthma.
Materials and methods This thesis rests on data from population-based databases and questionnaires: 1) The Norwegian Prescription Database, 2) The Norwegian Mother and Child cohort study questionnaire for seven years old participants, 3) The Youth Health Surveys in five counties, 4) The Population and Housing Census from 2001, and 5) The Central Population Register. Data on filled prescriptions from the Norwegian Prescription Database is the central source for information and provided the outcome variables in all papers of this thesis.
Main findings Mother-reported use of asthma drugs in children had high validity, compared to prescription data of asthma drug use. Furthermore, the prescription data on asthma drugs corresponded well with maternal reports of current physician-diagnosed asthma, and few individuals with no reported asthma had filled prescriptions. Filled prescriptions were used as a proxy measure in the other papers in this thesis to identify individuals with current asthma in the study populations. The prevalence and incidence of asthma drug use in the Norwegian population 2-29 years old was highest in preschool children and lowest in young adults. Males had higher levels than females at a young age, but this changed from about 15 years of age to higher levels in females. The persistence to asthma drugs over time was relatively low, with less than half of asthma drug users receiving drugs in three consecutive years. The type of asthma drugs used varied substantially by age but not by gender, and there were indications of suboptimal pharmacotherapy. The occurrence of chronic diseases in individuals with asthma was assessed by utilizing diagnostic codes provided by physician on reimbursed prescriptions. Several diseases occurred more frequently in children, adolescents and young adults with asthma than in the Norwegian general population. A majority of the asthma population had one of the nine comorbid diseases examined, while few had more than one of the comorbidities. In another study, young adults with asthma were at an increased risk for initiating use of hypnotic drugs. The risk was highest among individuals who recently had received asthma drugs, indicating that they had currently active asthma disease.
Conclusions The findings in this thesis indicate that prescription data may serve as a proxy for current asthma in the population. It is important to carefully consider the length of the capture period for prescription data and possible overlapping conditions treated with the same drugs in different age groups. The relatively low persistence to asthma drug use may reflect the variability of asthma within and between patients, and illustrate the challenges encountered in defining asthma in epidemiologic studies. Several diseases and drug treatments occurred more frequently in the asthma population. The presence of comorbidities may influence and complicate several aspects of asthma and both the causes and consequences of comorbidities need further investigation.
List of papers. Papers I-III are removed from the thesis due to copyright restrictions.
Paper I Furu K, Karlstad Ø, Skurtveit S, Håberg SE, Nafstad P, London SJ, Nystad W. High validity of mother-reported use of antiasthmatics among children: a comparison with a population-based prescription database. Journal of Clinical Epidemiology 2011;64:878-84. doi:10.1016/j.jclinepi.2010.10.014
Paper II Karlstad Ø, Nafstad P, Tverdal A, Skurtveit S, Furu K. Prevalence, incidence and persistence of anti-asthma medication use in 2- to 29-year-olds: a nationwide prescription study. European Journal of Clinical Pharmacology 2010;66:399-406. doi:10.1007/s00228-009-0749-x
Paper III Karlstad, Ø, Tverdal, A, Skurtveit, S, Nafstad, P, Furu, K. A prospective study of asthma and subsequent use of hypnotics in young adults. Pharmacoepidemiology and Drug Safety 2011;20:370-7. doi: doi:10.1002/pds.2101
Paper IV Karlstad Ø, Nafstad P, Tverdal A, Skurtveit S, Furu K. Comorbidities in an asthma population 8-29 years old - a study from the Norwegian Prescription Database. Manuscript. The definitive version is available at www3.interscience.wiley.com Pharmacoepidemiology and Drug Safety, Article first published online: 23 SEP 2011 doi:10.1002/pds.2233