Method and associated system for reading machine-readable labels on a plurality of sample receptacles held by a sample rack. In the method, a machine-readable label associated each of the plurality of sample receptacles is read with a first label reader when the rack is at a first location. The sample rack is then moved from the first location to a second location, where a rack identifier on the sample rack is sensed with a sensor separate from the first label reader. Finally, the rack identifier is associated with the machine-readable labels of the plurality of sample receptacles.