A method and apparatus for alerting a user of topical negative pressure therapy apparatus to a full waste canister condition are described, the apparatus comprising a device having vacuum pump means and a waste canister connected to the device and the waste canister operably connected to a wound dressing by aspiration conduit means for aspirating fluid from the wound, the aspiration conduit means, the waste canister and the device providing a fluid flow path therethrough and the vacuum pump means providing fluid flow through the apparatus, the apparatus further comprising fluid flow restriction means in the fluid flow path of said vacuum pump and fluid pressure sensing means upstream and downstream of said fluid flow restriction means.