Strengths and limitations
A major strength of this study is the use of routinely collected
prescription- and healthcare information in nationwide registers with
high completeness and data validity (32). Denmark has a longstanding
tradition for public registers and a universally tax-funded healthcare
system, which ensures coverage of the entire Danish population
regardless of differences in socioeconomic class or insurance status
(32). Due to OCS and other asthma drugs being prescription-only
medication, no potential over-the-counter exchanges were neglected.
However, several limitations much be acknowledged. In lack of access to
diagnostic data from primary care, we used medical prescription data as
proxies for asthma diagnoses. The method of identifying asthma patients
from prescribing data has been validated as a reliable method by several
European studies (19, 20). We chose a conservative upper age cut-off of
45 years to minimize the presence of COPD patients. We furthermore
excluded individuals diagnosed with COPD, cystic fibrosis or diseases
commonly treated with OCS in order to increase the probability of the
OCS use being attributed to asthma. The study design’s restrictions
might make our estimates more conservative, reflected in our finding of
a prevalence of active asthma at 3.4%. Also, the mildest cases of
asthma, requiring less than two asthma drugs per year, were not
identified. Due to the use of prescription data, we might have
overestimated the actual OCS use, as a dispensed prescription is not
synonymous to the medication being consumed. The use of dispensed
prescriptions, however, reduce the risk of misclassification due to
primary non-adherence. We were not able to account for possible
stockpiling. Data on the underlying indications for the prescribed OCS
were not available, and although we sought to describe only the OCS
usage in asthma treatment, OCS prescriptions for asthma patients are
often prescribed for other conditions (31). We sought to accommodate
this by excluding patients with several comorbidities likely treated
with OCS. In addition, we did not include OCS prescriptions filled
during periods of apparent inactive asthma as defined in the study
design, which possibly have led to an underestimation of the total OCS
exposure per individual.