Blood platelet aggregometer: predicted effects of aggregation, photometer geometry, and multiple scattering.

The in vitro aggregation of blood platelets is usually monitored with a visible light transmittance photometer (aggregometer). These cells in plasma are large (alpha = 2pi a/lambda approximately = 16) soft (m = 1.04) particles. The factors which significantly influence transmittance include the inherent scattering properties, multiple scattering, and photometer design. Now scattering theory and numerical methods for radiative transfer are used to survey how aggregation should influence transmittance as measured with various photometers. The results should help expand the analytical power of the transmittance photometer as a tool for monitoring aggregation. Evidence is also presented that scattering by aggregates of aerosol particles, which are of a higher relative refractive index, should also be adequately predicted by these approximate methods.