Bệnh nhân với đa hồng cầu vera rubra cũng tăng nguy cơ huyết khối khi hematocrits của họ là trên 55%. Huyết khối có thể là do các sự kiện tàu nhỏ, có lẽ một phần từ độ nhớt tăng lên, hoặc huyết khối mạch máu lớn. Bệnh nhân bị hội chứng myeloproliferative có nguy cơ cao hơn huyết khối thậm chí có số lượng máu tương đối bình thường | Bleeding and Thrombosis in Cancer Patients 193 Fig. . Endogenous erythoid colony assay. 27 count is in the 4-600 000 L range. Patients with polycythemia rubra vera are also at increased risk of thrombosis when their hematocrits are over 55 . The thrombosis may be due to small vessel events perhaps in part from increased viscosity or large vessel thrombosis. Patients with myeloproliferative syndromes have a higher risk of thrombosis even with relatively normal blood counts suggesting an intrinsic defect in the blood cells leading to thrombosis. Patients with myeloproliferative syndromes may have thrombosis in any location but thromboses at two certain sites should raise concern about an underlying myeloproliferative syndrome. Patients with Budd-Chiari and other visceral vein thromboses have a high incidence of underlying myeloproliferative syndromes. Patients with essential thrombocytosis can also have platelet occlusion of the small digital vessels leading to erthryomelalgia. These patients will have swollen red and very painful digits. The patients may only have slightly elevated platelet counts and are often misdiagnosed with arthritis. One helpful diagnostic clue is that these patients will respond dramatically to a single aspirin per day. Certain patients especially those with Budd-Chiari syndrome may have an occult myeloproliferative syndrome. Although there may be no evidence of any hematological disorder on the peripheral smear or bone marrow aspirate an established clonal proliferation of abnormal hematopoietic cells is present. A sensitive test for myeloproliferative disorders is the endogenous erythroid colony assay. This test depends on the ability of the abnormal clone to grow in culture without erythropoietin. A positive test can predate the onset of an overt myeloproliferative disorder by months to years. Up to 25-50 of patients with idiopathic Budd-Chiari syndrome will have a myeloproliferative disorder diagnosed by erythroid colony assay. The .
