The recommended and commonly used methods for the isolation of platelets from whole blood do not harvest a representative platelet population. There is evidence that these methods may result in the loss of a functionally more active platelet subpopulation. We describe a method whereby a completely representative population of platelets was isolated from the whole blood of 28 normal human volunteers by repeated washing of platelets from the red-cell layer. The harvesting efficiency was 98.3% +/- 2.8%. The platelets were labelled with 111In-oxine in a saline milieu with a labelling efficiency of 86.4% +/- 6.8%. The disappearance of reinjected labelled autologous platelets from the circulation was almost linear, and the mean platelet survival was estimated to be 224 +/- 23 h. At equilibrium, 61% +/- 12% of the labelled platelets were recovered from the circulation. The in vivo distribution at equilibrium and the sites of sequestration of the senescent labelled platelets were determined by geometric-mean whole-body quantification in six of the volunteers. This improved method permits accurate quantification of organ 111In radioactivity. Following reinjection, the labelled platelets pooled in the spleen and the accumulated activity can be presented by a single exponential function. At equilibrium, 31.1% +/- 6.1% and 9.6% +/- 1.2% of the platelets were in the spleen and liver, respectively. Splenic and hepatic radioactivity increased significantly with time, and at the end of the platelet life span, 35.6% +/- 9.7% and 28.7% +/- 8.3% of the labelled platelets were sequestrated in these organs, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The rate and extent of platelet mobilization from the spleen were measured, and their relationship to the removal of platelets from the peripheral blood during discontinuous flow platelet apheresis was determined in four normal volunteers. Autologous platelets were labeled with Indium-111-oxine and in vivo whole body and organ In-111 radioactivity quantitated with a scintillation camera and a computer-assisted imaging system. Dynamic changes in splenic radioactivity were monitored during 12 cycles of platelet apheresis. The number of platelets harvested and changes in whole body and blood In-111 activity were determined during the procedure. The platelet life-span was estimated, and the sites of sequestration of labeled platelets was measured. Platelet apheresis removed a mean of 64 percent of platelets in the circulation; i.e., 48 percent of all platelets in the body. During the procedures, 28.0 +/- 9.4 percent In-111-labeled platelets in the body were removed, splenic radioactivity decreased by 36.5 +/- 13.2 percent, and whole body activity decreased by 34.5 +/- 9.7 percent. In-111 activity in the spleen and whole body decreased in parallel, indicating a dynamic equilibrium between these pools. The life-span of the labeled platelets was 226 +/- 25 hours, similar to that of normal subjects. The major sites of sequestration of senescent platelets were the spleen (37.9 +/- 20%) and liver (30.3 +/- 5.6%); this is similar to that found in normal subjects. We conclude that as platelets are removed from the peripheral blood, the blood pool is rapidly and effectively replenished from the splenic platelet pool. These two pools are in dynamic equilibrium and permit removal of large numbers of platelets without resultant thrombocytopenia. Platelet apheresis does not adversely effect platelet life-span, and the sequestration pattern in the reticuloendothelial system is normal.

Mean platelet survival and turnover were simultaneously determined with autologous 111In-labeled platelets (111In-AP) and homologous 51Cr- labeled platelets (51Cr-HP) in ten patients with chronic immune thrombocytopenic purpura (ITP). In vivo redistribution of the 111In-AP was quantitated with a scintillation camera and computer-assisted image analysis. The patients were divided into two groups: those with splenic platelet sequestration (spleen-liver 111In activity ratio greater than 1.4), and those with diffuse sequestration in the reticuloendothelial system. The latter patients had more severe ITP reflected by pronounced thrombocytopenia, decreased platelet turnover, and prominent early hepatic platelet sequestration. Mean platelet life span estimated with 51Cr-HP was consistently shorter than that of 111In-AP. Platelet turnover determined with 51Cr-HP was thus over-estimated. The difference in results with the two isotope labels was apparently due to greater in vivo elution of 51Cr. Although the limitations of the techniques should be taken into account, these findings indicate that platelet turnover is not always normal or increased in ITP, but is low in severe disease. We suggest that this may be ascribed to damage to megakaryocytes by antiplatelet antibody. The physical characteristics in 111In clearly make this radionuclide superior to 51Cr for the study of platelet kinetics in ITP.

Six patients with giant platelet syndrome were examined: four with Bernard-Soulier syndrome (two were asplenic); one with hereditary thrombopathic thrombocytopenia; and one with May-Hegglin anomaly. Autologous platelets were labelled with In-111-oxine and in vivo redistribution and sites of sequestration measured with quantitative imaging. In Bernard-Soulier syndrome platelet survival was normal or moderately shortened; platelet turnover was decreased only in the two patients with thrombocytopenia. In the patients with thrombopathia or May-Hegglin anomaly, platelet survival and turnover was moderately decreased. In those patients with normal-sized spleens, the mean splenic platelet pool consisted of 35.5% of the platelet mass, i.e. normal. The intrasplenic transmit time of the megathrombocytes was prolonged. Splenic blood flow was within normal limits. There was a marked accumulation of platelets in the liver at equilibrium: 15.5-58.8% of whole body radioactivity (normal 9.6 +/- 1.2%). This finding is unexplained. The final sites of sequestration of platelets were mainly in the liver and spleen, similar to that seen in normal subjects. We conclude that there is no inverse relationship between cell size and splenic platelet transit time. Platelet size therefore does not determine the size of the splenic platelet pool. The size of the platelets also does not seem to affect the sites of sequestration at the end of their life span.