Providing a fluid to an instrument comprises supporting a sample receptacle on a first carrier, transporting the first carrier on a conveyor adjacent a plurality of modules including at least one instrument, stopping the first carrier at the instrument, transporting the sample receptacle to a pick-up position of the instrument, transporting the sample receptacle from the pick-up position to a pipetting station within the instrument, aspirating fluid within the sample receptacle and transferring the aspirated fluid to a reaction receptacle, after aspirating fluid from the sample receptacle, transporting the sample receptacle to the pick-up position, transporting the sample receptacle from the pick-up position to a second carrier the conveyor, in the instrument, performing an assay to determine the presence or absence of an analyte in the aspirated fluid, and transporting the second carrier on the conveyor to one or more of the plurality of modules other than the analytical instrument.