In addition detecting circulating tumour cells (CTCs) in blood, the presence of multi-cellular clusters has recently been identified to carry further information pertaining to patient outcome. We present a label-free method of measuring the range and load of clusters in a blood sample using a size-exclusion rail operated by centrifugal microfluidics. A negative selection strategy first enriches clusters from a whole blood sample; these clusters are then processed along the rail where they resolve to a series of collection bins according to size. Analysis of the occupancy of these bins then provides metrics on the cancer-cell load carried in the blood.