We have studied 16 normal subjects and 27 patients with stable, untreated thrombocytopenia secondary to bone marrow failure and platelet counts ranging from 12,000 to 70,000/microL. Autologous platelets were labeled with 51Cr for measurement of mean platelet life span in the normal subjects and in 20 patients. Labeled donor cells were used in the remaining subjects. Platelet survival, as determined with both autologous and homologous platelets, correlated directly with platelet count in the thrombocytopenic patients. Platelet life span was only modestly reduced in patients having counts in the range of 50,000 to 100,000/microL (7.0 +/- 1.5 days v 9.6 +/- 0.6; P less than .01) but was markedly reduced when the count fell below 50,000/microL (5.1 +/- 1.9 days, P less than .001). The recovery of donor platelets in severely thrombocytopenic recipients (60% +/- 15%) was equivalent to control values (66% +/- 8%; P greater than .2). The recovery of autologous platelets was normal when the platelet count exceeded 50,000/microL (74% +/- 15%) but was reduced in patients with lower counts (50% +/- 22%; P less than .01). All patient and normal data were well correlated by a model predicting a maximum platelet life span of 10 1/2 days and a fixed requirement for 7,100 platelets per microliter of blood per day, or about 18% of the normal rate of platelet turnover, which averaged 41,200 platelets per microliter per day. We conclude that although relatively few platelets are used to support vascular integrity, this requirement is reflected by a reduced platelet life span in marrow hypoplasia and may contribute to the shortening of platelet survival observed in other thrombocytopenias.

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