Geographical and sociodemographic disparities in oral corticosteroid burden and biologic use in difficult-to-treat asthma: a national population-based cohort study in New Zealand

Background Frequent oral corticosteroid (OCS) use for asthma causes serious side effects. International guidelines recommend steroid-sparing strategies such as biologics, but the geographical and sociodemographic variation of OCS and biologic use in difficult-to-treat (DTT) asthma across New Zealand is unknown. Methods We conducted a retrospective population-based cohort study using national datasets to map regional variation in DTT asthma, OCS burden, and biologic use. We also explored how sociodemographic factors affect OCS exposure. Individuals with DTT asthma were identified from community dispensing and hospitalisation data, where DTT was defined as high-dosage inhaled corticosteroids plus long-acting beta-agonist dispensings covering ≥90 days over 6 months. Individuals were defined as having uncontrolled DTT asthma if, in addition, they received ≥2 OCS dispensings or ≥1 asthma hospitalisation in 6 months. We mapped prevalence and OCS burden across geographic areas, adjusting for demographics using a zero-inflated negative binomial model that positioned DTT classification as an operational proxy for baseline disease severity. Findings Of 625 450 individuals with asthma, 100 017 (16·0%) had DTT asthma and 9261 uncontrolled DTT (9.3% of the DTT population). The uncontrolled DTT population was 66% female, 46% from low socioeconomic areas, 22% Māori, and 9% Pasifika. Among the overall population, 4.7% received ≥500 mg and 1.8% received ≥1 g cumulative OCS over a 12-month period; biologics use was 1·0% within this group. Small but consistent sociodemographic disparities persisted in OCS burden even after adjusting for disease severity. Rural areas had a lower likelihood of any OCS exposure but similar total OCS exposure. Interpretation This high-resolution geographical and sociodemographic analysis of DTT asthma, OCS burden and biologic use across New Zealand found marked regional disparities and a mismatch between disease prevalence and treatment. This suggests inequities in asthma care with a concerningly high rate of OCS use and a low rate of biologic use among those eligible.