Along with hearing aides themselves, related hearing aid supplies that are sold in Missouri are exempt from sales tax. Items considered supplies, such as drying kit, cleaning tools, and batteries, must be necessities for the maintenance and proper functioning of the aid itself. Taxpayers should note that battery testers are not considered a hearing aid supply as they are used to test the battery and not the aid itself and, therefore, will not be exempt from sales tax. (Missouri Department of Revenue, MO-Letter Ruling No. LR3715)