1. An apparatus for determining the presence of at least one target compound in a sample, comprising: a housing having a slot formed therein; a cartridge including a reaction strip having a sample region, a control region, and a background region located therein that is adapted for insertion into said slot; an optical imaging unit including an optical imager interconnected to a microcontroller positioned with said housing to capture images of said cartridge when inserted into said slot, and wherein said microcontroller is programmed to process said image to identify the location of said sample region, said control region, and said background region after said cartridge is inserted into said slot.