Light scattering changes caused by RBC aggregation: Physical basis for new approach to non-invasive blood count

We develop theoretical models of light transmission through whole blood considering RBC aggregation. RBC aggregates are considered to be the main centers of scattering in red/near-infrared spectral region. In pulsatile blood flow the periodic changes of aggregate geometry cause oscillations of light scattering. Thus scattering-assisted mechanism has to be taken into account in pulse oximeter calibration. In case of oversystolic vessel occlusion the size of aggregates grows, and the light transmission rises. Light diffraction on a single scatterer makes the transmission growth non-monotonic for certain spectral range. For the most typical set of aggregate parameters this range corresponds to wavelengths below 760 nm, and this prediction fits well both in-vivo and in-vitro experimental results. This spectral range depends on the refraction index mismatch and the geometry of aggregates. Both of them may be affected by the chemistry of blood. For instance, changes of glucose and hemoglobin have different effect on light transmission time response. Consequently, their content may be determined from time evolution of optical transmission.