The onset of asthma is often defined by patients? self-reported age at diagnosis in epidemiological studies. The validity of this might be questioned. Our objective was to evaluate the agreement between self-reported and registered age at asthma diagnosis and assess factors contributing to it.

As part of the FinEsS (Finland, Estonia, Sweden) respiratory survey in 2016, randomly selected population samples of 13,435 from Helsinki and 8000 from Western Finland were studied. Self-reported age at asthma diagnosis was compared to age at asthma diagnosis in the Finnish register on special reimbursement for asthma medication. The reimbursement right is based on physician-diagnosis compatible with GINA and Finnish guidelines. Furthermore, factors associated with the difference were evaluated.

Altogether 197 subjects from Helsinki and 144 from Western Finland had self-reported and registered asthma and data on age at diagnosis. Of these, 61.9% and 77.8% reported age at diagnosis within 5 years from the registered diagnosis. Median difference between self-reported and registered diagnoses in years was ?2 (IQR ?9 to 0) in Helsinki and ?1 (IQR ?4.25 to 0) in Western Finland indicating earlier self-reported age at diagnosis. The difference was smaller in participants that reported diagnosis within last 10 years. Subjects with difference of >5 years had higher tendency to allergic symptoms and more time had passed since the self-reported diagnosis.

The agreement between self-reported and registered age at asthma diagnosis is good, especially among patients with adult-onset asthma. Greater difference by patients with early-onset asthma could be related to delay in registration due to reimbursement criteria.