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Previous studies of platelet aggregation have shown that platelet disc-to-sphere transformation induced by ADP is accompanied by a decrease in light transmission. Our studies were directed towards an understanding of this effect; they also resulted in the development of a method which quantitatively reflected platelet shape. A Payton aggregometer with a 0.5 ml cuvette was used. Electronic settings were standardized to give a read out of 100% transmission (10 mV) on the recorder with water in the cuvette, and 0% transmission (0 mV) with background current alone. For fresh, discoid platelet suspensions there was a fall in light transmission to a minimum at 200 r.p.m. with an increase in transmission to a maximum at 700 r.p.m. with increasing stirring speed After disc-to-sphere transformation by ADP, EDTA, or chilling, a constant light transmission, un-effected by stirring rate, was observed. When the changes in light transmission due to the disc-to-sphere transformation were measured, only a small decrease took place at 0 r.p.m. At 200 r.p. m. there was actually an increase due to the shape change, while maximum decrease was obtained at 700 r.p.m. This shows that the platelet shape change observed in the aggregometer is mainly determined by the difference of discoid and spherical platelets in their effect on light transmission in presence of stirring. When the ratio O.D.700/O.D.200 was determined for 38 suspensions of fresh normal PRP kept at 37°C, with platelet count ranging from 0.05 - 0.4 × 106 per mm3, there was little variation (0.66± 0.04 S.D.). The ratio approached unity with spherical platelets. With spherical and discoid platelets mixed in various ratios O.D.700/0.D.200 could detect as little as 10% change in discoid platelets, demonstrating that this ratio gives a sensitive, objective and quantitative reflection of platelet shape.

A Simple, Quantitative, Objective Measurement of Platelet Shape