Summary
Extinction (E) or light transmission measurements of platelet suspensions have been
widely used to evaluate platelet structure and aggregation for platelet populations
of varying mean platelet size (V). However, useful comparisons of platelet suspensions
from donors having abnormally-sized platelets with those from healthy controls require
a knowledge of the dependence of E on V. We have analyzed the extinction per platelet
(e) as a function of the geometric scattering cross-section of equivalent spheres,
V2/3 , related to the apparent mean platelet optical efficiency, k. Such an analysis using
data previously reported by Holme et al., for healthy controls and a variety of platelet-associated
disorders showed e/V2/3 or k to be constant over an eight-fold variation in V. The outstanding exception
was platelet suspensions from myeloproliferative disorder patients (MPD) which showed
˜25% reduction in k values. When we analyzed e/V2/3 data for differentsized platelet subpopulations which were isolated by elutriation
from healthy donors, we found that k doubled with a doubling in V from small to large
platelets. It appears that the use of aggregometry devices with wide light acceptance
angles yields k values for platelets which are insensitive to variations in V, but
rather are sensitive to the expected variations in refractive index associated with
distinct platelet internal composition and organization expected for different-sized
normal subpopulations and “diseased” platelets as found in MPD. We relate these observations
to photometric studies of platelet heterogeneity and of aggregation of platelets with
different V and k values.
