An assay testing diagnostic analyzer system, comprising: a loading bay (36, 120) for loading a plurality of carriers (40, 126, 128) that are configured for holding containers (42, 122, 124) that contain fluid substances for use in a diagnostic process; a carrier positioner (80, 168) for receiving and positioning the carriers (40, 126, 128) for access by a diagnostic module to perform the diagnostic process; a transporter (50, 130) for transporting the carriers (40, 126, 128) from the loading bay (36, 120) to the carrier positioner (80, 168); and a retention member (176, 178) for coupling at least one of the carriers (40, 126, 128) to the carrier positioner (80, 168), characterized by the retention member (176, 178) including feet disposed on respective ones of the carriers (40, 126, 128) and stirrups disposed on the carrier positioner (80, 168), the respective feet to be releasably received in corresponding ones of the stirrups; and an activation member to control the retention member (176, 178) to one of lock the at least one of the carriers (40, 126, 128) to the carrier positioner (80, 168) or release the at least one of the carriers (40, 126, 128) from the carrier positioner (80, 168) when the transporter (50, 130) is in contact with the activation member, the activation member to engage a locking member, the locking member to be operatively displaced by the transporter (50, 130) to cause the feet of the retention member (176, 178) to move relative to the stirrups of the retention member (176, 178) to one of lock or release the at least one of the carriers (40, 126, 128) from the carrier positioner (80, 168). a loading bay (36, 120) for loading a plurality of carriers (40, 126, 128) that are configured for holding containers (42, 122, 124) that contain fluid substances for use in a diagnostic process; a carrier positioner (80, 168) for receiving and positioning the carriers (40, 126, 128) for access by a diagnostic module to perform the diagnostic process; a transporter (50, 130) for transporting the carriers (40, 126, 128) from the loading bay (36, 120) to the carrier positioner (80, 168); and a retention member (176, 178) for coupling at least one of the carriers (40, 126, 128) to the carrier positioner (80, 168), characterized by the retention member (176, 178) including feet disposed on respective ones of the carriers (40, 126, 128) and stirrups disposed on the carrier positioner (80, 168), the respective feet to be releasably received in corresponding ones of the stirrups; and an activation member to control the retention member (176, 178) to one of lock the at least one of the carriers (40, 126, 128) to the carrier positioner (80, 168) or release the at least one of the carriers (40, 126, 128) from the carrier positioner (80, 168) when the transporter (50, 130) is in contact with the activation member, the activation member to engage a locking member, the locking member to be operatively displaced by the transporter (50, 130) to cause the feet of the retention member (176, 178) to move relative to the stirrups of the retention member (176, 178) to one of lock or release the at least one of the carriers (40, 126, 128) from the carrier positioner (80, 168).