A carrier for holding up to N test sample devices as they are moved through a sample testing instrument. Each of the test sample devices are held in a receiving structure such as a slot in the carrier. The carrier also includes N optical interrupt positioning features, each placed in registry with one of the receiving structures (and thereby in registry with the test sample device). The instrument includes fixed optical interrupt sensors for detecting the position of the positioning feature as the carrier is moved through the instrument. In the illustrated embodiment, the position features comprise voids formed in a rib on the lower surface of the carrier. The optical interrupt sensors are positioned below the path the carrier travels over, whereby as the carrier moves past the sensor the voids, and hence position of the test sample devices, are detected.