Progressive functional impairment is known to occur in platelet concentrates through the storage period. Standardized methods providing direct measurement of residual platelet function in stored platelets are lacking. The purpose of this study was to determine whether a new platelet function analyzer (PFA-100 ) could provide standardized methods for assessing the hemostatic capacity of stored platelets.The PFA-100 was used to evaluate platelet function in stored platelets. The instrument can process citrated whole blood but it is unable to process platelet suspensions. Accordingly, the function of platelet concentrates should be measured following reconstitution of pseudo-whole blood. The analysis of the results included the closure time (sec) and a predictive index, an arithmetical index computed on the basis of the instrument's output data: the flow rate, the flow volume, the closure time.A final hematocrit of 58+/-2 and a final platelet concentration of 230+/-20x10(9)/L were used as standardized operative conditions to measure the function of stored platelet concentrates. The closure time (PFA-CT) and the predictive index (PFA-PI) both resulted to be capable of discriminating platelet concentrates with maintained or impaired function. PFA-PI was more informative than PFA-CT in terms of description of the residual platelet function. Of the two agonists used, epinephrine (EPI) resulted to be particularly sensitive for the detection of initial platelet hyporeactivity, whereas adenosine 5'-diphosphate (ADP) was particularly useful for measuring the residual platelet reactivity.PFA-CT and PFA-PI can be standardized; they provide new information about the hemostatic function of stored platelet concentrates and can be used to assess the quality of platelet concentrates.