Much improvement has taken place in evaluating sources of variability in the measures of delivered dose uniformity (DDU) and aerodynamic particle size distribution (APSD). Methods described in the United States Pharmacopeia (USP) and European Pharmacopoeia (Pharm. Eur.) now include dimensions and diagrams for all components critical to their robustness. However, we show that there are sources of bias in the standard methods for DDU. A future article will address APSD.
Control of the volumetric flow rate into sampling apparatuses for the performance testing of all types of passive dry powder inhalers (DPIs) is critical, as this measure influences the kinetics associated with mass transfer of the active pharmaceutical ingredient(s) (API(s)) from the inhaler to the measurement apparatus. However, and perhaps surprisingly, volumetric flow rate is a dependent variable in the standard pharmacopeial protocols. Given this situation, it is incumbent for stakeholders to be aware of sources of potential bias to have complete confidence in this important inhaler performance measure.