Platelet function and morphologic characteristics were evaluated in 43 patients with myeloproliferative disease (MPD), 5 patients with myelodysplastic syndrome (MDS), and 7 patients with secondary thrombocytosis (ST). Platelet Factor IV (PF4) and Bthromboglobulin (BTG) showed slight elevation in ST but significant elevation in all MPDs. They were either normal or slightly elevated in MDS. Defective platelet aggregation with one or more inducers was seen in 62% of all patients. An

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... uced defect was the most consistent aggregation abnormality. Hyperaggregation and spontaneous aggregation were seen in 15% of patients. Of the eight patients who showed increased bleeding tendency, all eight showed defective aggregation with two or more inducers, five showed decreased surface activation response, as well as decreased or abnormal granules and dense tubular disarray in the transmission electron microscope (TEM) study. Seven patients had clinical evidence of recurrent arterial and venous thromboses. Five of these patients showed hyperaggregation response to adenosine diphosphate and collagen and abnormal Wu and Hoak platelet aggregate ratio. Four patients showed spontaneous aggregation on aggregometer. Surface activation response was significantly increased in five patients, and an increase in platelet granules by TEM study was seen in four patients. Primary thrombocythemia could be differentiated from secondary thrombocytosis (ST) by the presence of abnormal aggregation response and significantly increased PF4 and BTG in the former, and greatly elevated plasma fibrinogen and Factor VIII, as part of acute phase reactant response, in the latter. (Key words: Myeloproliferative disease; Myelodysplastic syndrome; Secondary thrombocytosis; Platelet function; Platelet structure) Am J Clin Pathol 1989;91:647-655 BLEEDING AND THROMBOSIS are commonly seen in patients with chronic myeloproliferative diseases (MPDs). These complications usually have been ascribed to the frequently encountered thrombocytosis and increased red blood cell mass. Recent studies have reported a variety of functional and morphologic abnormalities of platelets and megakaryocytes, which also can produce symptoms in MPD. 4,5 A comprehensive review of many of these studies was published in 1984 by Schaefer. 8 The following investigation represents an evaluation of the functional as well as the morphologic aspects of platelets in a large group of patients with various MPDs, my-