A time-dependent increase in the responsiveness of platelet-rich plasma to epinephrine.

The variable responsiveness of platelet-rich plasma (PRP) to epinephrine and collagen was studied. These studies revealed that as PRP is allowed to stand at room temperature it becomes progressively more responsive to the aggregating effects of epinephrine and collagen. Samples of PRP obtained from 10 normal subjects showed only 8 plus or minus 8 per cent aggregation when tested after 5 minutes with "near threshold" concentrations of epinephrine. After 45 minutes the addition of the same concentrations of epinephrine to portions taken from the same samples of PRP yielded 72 plus or minus 12 per cent aggregation. Although similar results were obtained with "near threshold" concentrations of collagen, the variability of responsiveness to collagen could be easily overcome by increasing the collagen concentration. Such was not the case with epinephrine, and, a time-dependent acquisition of responsiveness was demonstrable over a broad range of epinephrine concentration. These findings suggest that variations in the amount of platelet aggregation observed with epinephrine may be a reflection of differences in the time factor involved in testing. This time factor should be taken into account before epinephrine-induced platelet aggregation curves are compared and defined as "normal" or "abnormal".