Background: Severe asthma is a considerable challenge for patients, health care professionals and society, but there are few long-term studies on risk factors for severe asthma.

Aim: To identify baseline risk factors of severe asthma in a longitudinal adult asthma cohort study.

Methods: An adult asthma cohort was identified in 1986-2001 by clinical examinations of population samples within the OLIN studies in northern Sweden. The examinations included structured interviews, spirometry with reversibility testing, skin prick testing and metacholine challenge. The cohort was followed up in 2012-2014 when n=1006 participated (mean age 59y). Adjusted Risk Ratios (RR) for baseline factors as risk factors for GINA defined severe asthma (SA) at follow-up (n=51) were estimated by Poisson regression.

Results: Older age, impaired lung function, increased reversibility and hyperreactivity, asthmatic wheeze, persistent wheeze, nighttime awakenings due to respiratory symptoms, and dyspnea were significant baseline risk factors for SA. Allergic sensitization, smoking, occupational groups or BMI did not predict SA. When adjusted for age, sex and smoking, post-bronchodilator FEV1/FVC<0.7, both present at baseline (RR 4.2, 95%CI 1.8-9.9) and developed during follow-up (2.9, 1.6-5.3), increased the risk. Also FEV1<80% at baseline associated with SA (2.9, 1.6-5.2). Triggers for respiratory symptoms at baseline such as tobacco smoke (2.1, 1.2-3.7) and physical activity (3.5, 1.5-81) associated with SA at follow-up, while pollen and furry animals did not.

Conclusion: Among adults with asthma, impaired lung function, wheeze, dyspnea and nighttime awakenings due to respiratory symptoms are important long-term risk factors for severe asthma.