An apparatus adaptable for use in a centrifuge apparatus to rotate a fluid tube about its longitudinal axis while the centrifuge apparatus is rotating the fluid tube in a rotational direction transverse to the longitudinal axis of the tube. The apparatus comprises an engaging member configured to engage a gear which is mechanically coupled to the fluid tube, and a driver configured to apply a driving force to the engaging member to cause the engaging member to engage and rotate the gear, which rotates the fluid tube about its longitudinal axis. The centrifuge apparatus can therefore obtain readings of the centrifuged sample from different locations about the circumference of the fluid tube at different orientations of the fluid tube.