Abstract: A quantitative measuring instrument includes a casing with a cavity for a reagent strip handling mechanism. The mechanism includes a slide with a resiliently biased toggle member for providing snap action of the slide between an open, reagent strip loading position and a closed, quantitative measuring position. A light seal is mounted on the toggle member that engages each reagent strip in the closed position of the slide. In a first, alternative embodiment of the instrument, the slide mechanism includes a cam arm that engages cam surfaces on the instrument raising the cam arm to an open, reagent strip loading position from a closed, reagent strip holding position. In this embodiment the slide can be removed from the instrument and cleaned. In a second alternative embodiment the slide includes a first detent corresponding to a closed position of the slide and a second detent corresponding to an open position. A pair of spring fingers are mounted in the cavity of the instrument casing.