Platelet kinetics

Platelet kinetics can be subdivided into three topics: distribution, particularly with respect to the liver and spleen; life span; and sites of destruction. Following injection, a substantial fraction of radiolabelled (111-In or 113m-In) platelets are taken up by the spleen. The normal platelet survival is linear when determined using 111-In labelled platelets, with a life span of about 9-10 days. Using 51-Cr it has been found by some to be linear whereas by others to be curvilinear or truly exponential, creating confusion as to exactly what is the life span with 51-Cr labelling. Quantitative scanning over the liver and spleen following injection of 111-In labelled platelets has shown that, initially, the sum of total activities in liver, spleen and blood is equal to 100% of the injected dose. This sum falls progessively throughout the lifespan of the radiolabelled platelets, and, because (i) there are no losses of 111-In via urine or faeces and (ii) a bone marrow image becomes increasingly visible, it is likely that the rising 111-In ''defecit'' is present in bone marrow. Since 111-In is stable in the spleen and liver, the increasing bone marrow activity probably represents uptake of platelets rather than of ''free'' 111-In. Based on this approach, it was concluded that approximately as many platelets are destroyed in the bone marrow as in the spleen - about 40% to 50%. The remainder, about 15% appear to be destroyed in the liver, but, because a significant fraction of this probably represents immediate uptake of platelets damaged by the labelling procedure, it must be concluded that the liver is not normally an important site for platelet destruction. (Auth.)