Introduction: In 2010, 6.8% of the Portuguese adults had asthma. Contemporary studies employing more accurate methods are needed. We aimed to assess asthma prevalence in Portugal and to identify associated-factors.

Methods: A population-based nationwide study was conducted from May 2021 to March 2024. A multistage random sampling approach was applied to select adults from primary care. Stage 1 involved a telephone screening interview to collect socio-demographic and clinical data. Patients with an Adult Asthma Score (A2 Score) >= 1 were eligible for Stage 2, and 5% of those with an A2 Score = 0 were also invited to participate in Stage 2, which consisted of a diagnostic visit with a physical examination and diagnostic tests. We computed weighted asthma prevalence estimates and multivariable logistic regression models were used.

Results: A total of 7,556 participants completed Stage 1 and 1,857 Stage 2. The prevalence of asthma was 7.1% (95%CI = 6.3-8.0%), with slight differences by sex, age, and region. Education, family history of asthma, inhaler prescription, nasal/ocular symptoms, food allergies, and previous allergy skin tests were associated with an increased risk of asthma (R2 = 33%). Asthma diagnosis could also be predicted by the A2 score, either on its own (R2 = 43%) or in combination with family history and previous allergy skin tests (R2 = 45%).

Discussion: Asthma affects 7.1% of Portuguese adults. Family history of asthma, nasal/ocular symptoms, and comorbid food allergy are associated with increased risk of asthma.