A breath tester housing assembly is provided. The assembly comprises a housing having a base to be gripped by an operator, a display oriented to be in line with an operator's direct line of view while gripping the base, and a mouthpiece interface for interfacing with a removable mouthpiece so that when a subject blows into the mouthpiece, the display is not in the direct line of view of the subject. The assembly further comprises a mouthpiece for engaging to the mouthpiece interface. The mouthpiece comprises a body having a substantially planar surface.