Red Blood Cells flowing in a microchannel undergo dispersion in the flow direction due to the nonuniform velocity profile while transverse migration due to cell-wall interactions tends to focus them along the center line. This results in a dispersion of RBC transit times through a capillary that is directly related to their transverse migration properties. By analogy with the Taylor-Aris problem, we present an experimental method to characterise this phenomenon by injecting pulses of red blood cells and measuring the evolution of their length along the channel, and varying mechanical parameters such as RBC deformability and fluid viscosity. A direct comparison of experimental results with a model that incorporates longitudinal advection and transverse migration shows that this principle provides through a simple dispersion measurement an evaluation of migration characteristics that are directly connected to cell mechanical properties.