A method of configuring a hand-held instrument (10) for determining the concentration of a medically significant component of a body fluid or a control comprises providing a configuring computer (14) having a first port (18) for transmitting instructions and data for configuring the instrument (10), providing on the instrument (10) a second port (17) for receiving the instructions and data from the configuring computer (14), coupling the first port (18) to the second port (17), transmitting instructions and data to configure the instrument (10) from the first port (18), receiving the instructions and data at the second port (17), and configuring the instrument (10) according to the instructions and data transmitted from the first port (10) and received at the second port (17).