During storage, human platelet concentrates progressively lose the capacity to survive and function in vivo after transfusion. A shape transformation from disc to sphere is the most reliable in vitro determinant for the loss of the in vivo survival of platelets. To find an objective measurement for platelet morphology, we studied the effect of anticoagulant, temperature, and storage on the apparent median platelet volume (MPV) as determined by a particle counter and on changes in platelet shape as measured and by light microscopy. Changes in MPV, light transmission, and morphology score by light microscopy were observed within 1 minute after collection of blood in CPDA. As compared to blood immediately fixed on withdrawal, in CPDA blood, the MPV increased from 4.1 to 5.7 fl, and light transmission difference decreased from 22 to 7 percent. A partial restoration of these determinants was found when the whole blood was incubated for 30 minutes at 37 degrees C, before preparation of platelet-rich plasma. In the first 5 days of platelet storage, the MPV increased from 4.6 to 5.0 fl; thereafter, it started to decrease. An increase in fragmented platelets after 5 days was observed on light microscopy. The light transmission difference showed a slow disc-to-sphere transformation during storage. This transformation accelerated from Day 5 to Day 7; after 11 days, only spheres were detected. After 7 days the swirling pattern scores were still in accordance with the presence of discs, whereas the other structure-associated determinants showed already spheric and even fragmented platelets.(ABSTRACT TRUNCATED AT 250 WORDS)