Comparing weight method and float method for pressurised metered dose inhaler as dose counting method.

INTRODUCTION Pressurised metered-dose inhalers (pMDIs) are not equipped with dose counters, so estimating the actuation remaining is difficult. We compared weight method and float method in estimating the actuation remaining in discarded pMDIs.METHODOLOGY Used non-metered pMDIs were collected from Pharmacy department Hospital Seri Manjung from June to Dec 2018. Each of the returned canister was evaluated using float and weight method to predict the balance actuation in the canister. For the float test, the canister was placed in a container of water. Based on its position in the water, the pMDI was categorised as 1=empty, 2=¼ full, 3=½ full, 4=¾ full, or 5=full. For the weight method, canisters were weighed, and actuation remaining were calculated using a linear regression equation where the general equation is: Actuation remaining = Constant + β*pMDI weight, and further categorised as above. The percentage of empty pMDIs between the two methods were compared.RESULTS A total of 4517 pMDIs were returned to the pharmacy, of which 2131 (47%) were salbutamol, 1149 (25%) budesonide, 685 (15%) ipratropium/fenoterol, 340 (8%) innovator fluticasone, 182 (4%) generic fluticasone, and 30 (1%) beclomethasone. Overall, the float method classified 75% pMDIs as empty, compared to 71% by the weight method. The percentage of empty salbutamol, generic fluticasone, and beclomethasone pMDIs were similar between both methods. A higher percentage of budesonide pMDIs were classified as empty by the float method compared to the weight method (62% vs 37%). For ipratropium/fenoterol pMDIs, the float method did not identify any empty canisters, but 82% were classified as empty by the weight method. CONCLUSION This study produced a prediction equation that can be used to estimate remaining actuation in a pMDI based on its weight. There were differences in the identification of empty pMDIs between the float test and the weight method, which varied by drug. The weight method may be a convenience method to be used to measure actuation remaining in pMDIs returned to the pharmacy, as well as patients’ adherence to pMDIs.