The laboratory diagnosis of immune platelet destruction has relied predominantly on the presence or absence of megakaryocytes in bone marrow. Recently, examination of peripheral blood platelets for high
RNA content (reticulated platelets) or for elevated levels of platelet-associated
IgG have been suggested as less invasive diagnostic tests. We used
thiazole orange fluorescence labeling to determine the percentage of circulating reticulated platelets and two
antibodies with different specificities directed against human
IgG to measure platelet-associated
IgG by flow cytometry in 59 patients with either
immune thrombocytopenic purpura (n = 23) or
chemotherapy-induced
thrombocytopenia (n = 36). The percentage of reticulated platelets in patients with
immune thrombocytopenia was significantly increased (38.6% +/- 27.4% [mean +/- 1 SD]), compared with patients receiving
chemotherapy and normal subjects (7.2% +/- 3.3% and 2.9% +/- 2.2%, respectively). However, 17% of patients with
immune thrombocytopenia had reticulated platelet values in the range observed for normal subjects and for patients with
chemotherapy. Although one third of patients with
immune thrombocytopenia had very high platelet
IgG levels, the majority could not be distinguished from patients receiving
chemotherapy solely on this basis. Combining the reticulated platelet determination with the
IgG data did not improve the sensitivity or specificity of the reticulated platelet determination alone. We conclude that a flow cytometric assay for reticulated platelets is a better discriminant than flow-measured platelet
IgG for diagnosing immune platelet destruction. We further postulate that the subset (17%) of patients with immune destruction who have relatively low percentages of reticulated platelets may represent patients with an inappropriately low thrombopoietic response.