A Kinetic Model for Human Platelet Survival with Long‐Lasting Time‐Activity Curves

A model is presented that allows for the interpretation of the time course of the level of radiolabeled platelets in terms of platelet survival times, rate constant for removal from circulation, pooling time in an extra pool, the rate at which platelets re‐enter circulation from the extra‐pool, and the size of the plasma pool and extra‐pools. The tenets of the model are that: (1) platelets leave the circulation at a rate proportional to their number per unit volume; (2) of the leaving platelets, a fraction b goes into a pool from which they return into circulation after a pooling time and another fraction (1 ‐ b) is irreversibly destroyed; (3) the platelets in the extra‐pool do not “queue up”, and thus the distribution function describing the probability of return is exponential; and (4) the time activity curve of the radiolabeled platelets can be described by the sum of two exponentials. Under steady state conditions, curve fitting allows determination of the constants determining the time activity curve (the respective amplitudes and rate constants of the two exponentials); mean pooling time; relative pool size; and survival time of platelets. The model is applied to data collected from patients over a period from 10 days following reinjection of autologous radiolabeled platelets.