THE INVIVO KINETICS OF IN-111 LABELED AND CR-51 LABELED PLATELETS - A COMPARATIVE-STUDY USING BOTH STORED AND FRESH PLATELETS

The in vivo kinetics of simultaneously injected Cr-51- and In-111-labelled platelets was investigated in 20 healthy male volunteers. The studies were carried out using both fresh platelets and platelets stored for 5 d at 22-degrees-C; the disappearance of platelet-bound radioactivity was measured on whole blood samples as well as platelet pellets. Compared to In-111, the labelling efficiency was notably lower for Cr-51, and a higher amount of Cr-51 was bound to contaminating red cells. As regards the fresh platelets, only small dissimilarities were observed in the in vivo kinetics obtained with the two labels. Cr-51 yielded a slightly higher platelet recovery and longer T1/2 than In-111, when whole blood samples were used for calculations; no differences were seen when using platelet pellets. When stored platelets were studied, Cr-51 gave significantly longer T1/2 and mean life-span (MLS) than did In-111. This difference was present for all mathematical models used for the calculation of MLS, and when whole blood samples as well as platelet pellets were employed. It was demonstrated that the estimation of MLS was also highly dependent upon techniques of blood sampling and curve fitting. Calculations, in which the initial data points were excluded, gave consistently longer MLS (P < 0.0001). as compared to when all data points were included. Furthermore, when all survival studies were grouped together, calculations using platelet pellets gave a significantly (P < 0.001) shorter platelet MLS than calculations using whole blood samples. It is concluded that both Cr-51 and In-111 are acceptable as radiolabels for both fresh and stored platelets. However, it appears that the viability of stored platelets may be influenced by the choice of label, and caution must be taken when these isotopes are used together in dual tracer experiments. Also, our results show that a meticulously standardized processing of blood samples and experimental data is required to enable interlaboratory comparisons of the results.