A desktop stapler uses a spring to store energy to install staples by impact blow. A piercing member is adapted to pierce and retract from a site on the patient to cause the fluid to flow therefrom, and the sensor is positioned adjacent to the site on the patient so as to receive the fluid flowing from the site to generate an electrical signal indicative of the concentration of the analyte in the fluid. The end plate has a recessed portion located between the outer end portion which projects into the air chamber to reduce the volume of the air chamber to change the spring characteristic of the air spring. The metal injection molded sole portion preferably has a mass ranging from 45 grams to 110 grams.