The Global Usability Score Short-Form for the simplified assessment of dry powder inhalers (DPIs) usability

BACKGROUND: The choice of the Dry Powder Inhaler (DPI) to prescribe is a critical issue. The estimation of DPIs usability depends on the objective assessment of several indices related to both subjective and objective determinants. The Global Usability Score (GUS) Questionnaire is a comprehensive tool usable for checking, comparing, and ranking inhalers’ usability objectively in real life, but it takes some time to fill. AIM: The aim of this study was to favour the quicker check of DPIs usability in clinical practice by means of a simplified short-form GUS (S-GUS) Questionnaire, while maintaining the high specificity and sensitivity of the original, extended version of the Questionnaire (O-GUS questionnaire). METHODS: The usability of the six most prescribed DPIs was assessed in 222 patients with persistent airway obstruction and needing long-term inhalation treatments. LASSO regression and multicollinearity test were used to select the subset of questions of the O-GUS questionnaire, with the highest information power. Each item was then scored using the corresponding coefficient in the linear regression (normalized at 50 as the O-GUS score). Agreement between the original and the short-form questionnaire was evaluated using the Cohen’s kappa statistic (κ). The overall S-GUS values obtained for each DPI were then compared to those from the O-GUS, in the same patients, using a Bayesian indirect comparison (IC) model. RESULTS: After the statistical selection of the items mostly contributing to the overall score, the novel S-GUS questionnaire consists of twelve items only. Nine items are related to patients’ opinion before DPIs handling, and three to the nurse’s assessment after DPIs practicality. O-GUS and S-GUS score were strongly correlated (R(2)=0.9843, p<0.0001) and the usability score calculated for each DPI by means of the O- and of S- GUS overlapped almost completely (κ=84.5%, 95% CI 81.3% to 89.2%). Furthermore, S-GUS was much faster to complete than O-GUS (mean time 6.1 vs 23.4 minutes, p<0.001). Estimates of S-GUS, obtained from the IC model, allowed to propose a simple classification of usability: “good” by GUS values >25; “pretty good” by values ≤25≥15, and “insufficient” by values <15. CONCLUSIONS: The S-GUS proves as much specific and suitable as the extended O-GUS questionnaire in measuring DPIs usability, while maintaining the same high sensitivity. As the time required for its use is quite shorter, S-GUS is also particularly suitable and helpful in current clinical practice.