Introduction: There is a paucity of research on longitudinal lung function in severe asthma.

Aim: This study retrospectively evaluates lung function trajectories in the Birmingham Regional Severe Asthma Service.

Methods: Longitudinal spirometry from 140 patients with confirmed severe asthma (107 female, 33 male) were analysed retrospectively. Slopes for forced expired volume in one second (FEV₁) over time were assessed using linear regression. After measuring median slope within the cohort, sub-analysis was performed to investigate lung function patterns in those with most severe airflow obstruction (FEV₁ <50% predicted). Reference values for lung function were based on Global Lung Initiative prediction equations. 

Results: Median follow-up duration was 92 months (83-118), with a median of 10 spirometry measurements per person (range: 3-36). Sixty-one patients (43.6%) showed an increasing FEV₁ regression slope and 79 (56.4%) showed declining lung function. Median decline in FEV₁ was -30 mL·y^-1 (IQR: -55 to +24) for males and -6 mL·y^-1 (IQR: -44.5 to +28.5) for females. Thirty-five of 140 patients (25%) showed a rapid decline in FEV1 ?50 mL·y^-1 during follow-up. Fifty-two patients (37.1%) had severe impairment of FEV₁ at baseline (<50% predicted). Of these individuals, 9 showed a meaningful improvement in lung function (regression slope +50 mL·y^-1), 33 remained stable, and 10 continued to decline rapidly.   

Conclusion: Patients with severe asthma show distinct patterns of lung function over time. A quarter of patients show rapid decline in FEV₁ which may contribute to poorer health outcomes, particularly in those with already severe impairment of baseline spirometry.