An interface for a microfluidic device such as a microfluidic cassette has a seal portion moveable from a first position to a second position within a reservoir in response to an applied force. Movement of the seal portion within the reservoir causes the volume of the reservoir to increase. The interface also has a hollow needle with a first end and a second end, the first end locatable in fluid communication with the reservoir and the second end located outside of the reservoir and arranged for piercing. The seal portion and the needle are arranged such that when the seal portion moves from the first position to the second position, a predetermined volume of fluid is drawn through the needle into the reservoir.