The present invention relates to a hematology instrument system that discriminates platelets from red blood cells, debris and other particles within a blood sample. The instrument system uses an optical trigger and collects data at optical sensor locations relative to a flow cell-illuminating laser beam's optical axis. An axial sensor measures axial light loss due to a particle in the flow cell's illumination aperture. In addition, the instrument system can include an RF unit generates electrical parameters, such as DC and RF parameters, within the flow cell. Blood specimens can be prepared with and without a sphering agent.