Background and Aims: Bronchiectasis is a chronic respiratory condition, characterised by recurrent infection and exacerbations. In the Indigenous Australian population of the Northern Territory, the prevalence of bronchiectasis is the highest in the world. Bronchiectasis also appears earlier, with a greater range of comorbidities, different micro-organisms and earlier mortality compared to what is seen among non-Indigenous populations. As such, the existing severity scoring tools display poor efficacy in this population. The IBAS was developed to fit the need for accurate scoring of bronchiectasis among indigenous populations.

Methods: A retrospective cohort of 459 Indigenous Australian adult patients was enrolled from 2011 through 2020, with up to 5-years mortality and respiratory hospitalisation follow-up. Patients with excessive hospitalisations (> 100) or < 5 year mortality data were excluded.

Results: 454 patients (n = 253 female (55.7%)) were included. Females compared to males were similar age (median 47.2 (IQR 38.9, 56.5) vs. 46.6 (38.5, 55.6)) and BMI (median 24.3 (IQR 19.5, 27.1) vs. 22.5 (19.0, 25.7)) with similar prevalence of COPD (8.18 vs. 83.6%), current smoking (41.3 vs. 44.3%) and urban residence (7 vs. 8%). Percent predicted FVC was marginally lower (49 (IQR 36, 63) vs. 54 (IQR 45, 65.5)), FEV1 similar (38 (IQR 26, 52) vs. 37.1 (IQR 28.6, 52.5)) and FEV1/FVC greater (0.68 (IQR 0.54, 0.79) vs. 0.62 (0.45, 0.71)). 5-year mortality was 14.6 & 14.4% for females and males respectively. The IBAS risk score distribution, accounting for missing information, was similar between females and males (mild: 45.1% and 48.3%, moderate: 38.7% and 35.8%, severe: 16.2% and 15.9%). Area under the curve analysis showed a slightly greater predictive capacity for 5-year mortality via the IBAS risk categorisation in females (0.768 (95% CI 0.698, 0.839)) than males (0.729 (95% CI 0.635, 0.823)). Sensitivity and specificity against 5-year mortality of a moderate risk score were 91.9% and 51.4% for females vs. 82.8% and 53.5% for males, while for a high- risk score were 43.2% and 88.4% vs. 41.4% and 88.4% respectively.

Conclusion: The IBAS showed good accuracy for predicting 5-year mortality in both female and male Indigenous Australian adult patients. A Moderate risk score appeared to have higher sensitivity among females than males, however this aside there were no significant differences in the scoring and utility of the IBAS.