An analytical model of laser beam scattering on an inhomogeneous ensemble of red blood cells is constructed. The model takes into account the difference in red blood cells in size, shape, and orientation in space. The connection of the experimentally measured parameter – the visibility of the diffraction pattern – with the characteristic that has the meaning of a measure of the heterogeneity of the blood sample in terms of the size and shape of red blood cells is established. The monotonicity property of this dependence is shown, which makes it possible to estimate the spread of red blood cells by size and shape based on the measurement of the visibility of the diffraction pattern.