A microfluidic device for testing a fluid, the microfluidic device having an inlet for receiving the fluid, a reservoir containing a
reagent, a flow-path extending from the inlet, a plurality of outlet valves for fluid communication between the flow-path and the reservoir, each of the outlet valves having an
actuator for opening the outlet valve in response to an activation
signal, wherein during use, a number of the outlet valves are selectively opened such that the
reagent flows into the flow-path to combine with the fluid from the inlet to produce a combined flow having a proportion of the
reagent, the proportion of the reagent in the combined flow being determined by the number of the outlet valves opened.