Changes in platelet shape and volume as produced by low temperature, various anticoagulants and during storage of platelet concentrates (PC) were studied. Platelet volume was estimated by the mean platelet size, M, as measured by Coulter counter, and by the extinction (optical density) per platelet, e, as measured by a Payton aggregometer. For fresh PRP from 31 normal donors e and M were positively correlated (r = .88). Platelet disc-to-sphere transformation was quantitated as described

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... ly (J. Lab. CI in, Med. 92:53, 1978). A linear correlation between the degree of shape change and the apparent increase in size as reflected by M and e was found for citrated PRP exposed to low temperatures and ADP. PRP obtained from blood anticoagulated with EDTA and kept at 22°C for 1-3 hours had increases in H that exceeded what could be predicted by shape change alone and a proportionate decrease in e. Light scattering theory would predict this if cellular swelling had occurred. When PC are stored for transfusion at 22, they are commonly exposed to hypoxic conditions (Blood 46:209, 1975) which lead to a pH fall and disc-to-sphere transformation. If pH remainsistable, there is a 10% decrease in M and e after 3 days. If pH falls below 6.7, there is a progressive increase in M beyond what can be accounted for by shape change and a proportionate decrease in e. This swelling could be partially but not completely corrected by incubation in oxygenated fresh plasma at 37°C. We suggest that this swelling reflects cellular damage which results from hypoxia.