Bleeding time and platelet function in essential thrombocythemia and other myeloproliferative syndromes

Platelets Contribution to Thrombin Generation in Philadelphia-Negative Myeloproliferative Neoplasms: The "Circulating Wound" Model

Current cytoreductive and antithrombotic strategies in MPNs are mostly based on cell counts and on patient's demographic and clinical history. Despite the numerous studies conducted on platelet function and on the role of plasma factors, an accurate and reliable method to dynamically quantify the hypercoagulability states of these conditions is not yet part of clinical practice. Starting from our experience, and after having sifted through the literature, we propose an in-depth narrative report on the contribution of the clonal platelets of MPNs-rich in tissue factor (TF)-in promoting a perpetual procoagulant mechanism. The whole process results in an unbalanced generation of thrombin and is self-maintained by Protease Activated Receptors (PARs). We chose to define this model as a "circulating wound", as it indisputably links the coagulation, inflammation, and fibrotic progression of the disease, in analogy with what happens in some solid tumours. The platelet contribution to thrombin generation results in triggering a vicious circle supported by the PARs/TGF-beta axis. PAR antagonists could therefore be a good option for target therapy, both to contain the risk of vascular events and to slow the progression of the disease towards end-stage forms. Both the new and old strategies, however, will require tools capable of measuring procoagulant or prohaemorrhagic states in a more extensive and dynamic way to favour a less empirical management of MPNs and their potential clinical complications.

Unexpected low expression of platelet fibrinogen receptor in patients with chronic myeloproliferative neoplasms: how does it change with aspirin?

This study was conducted to evaluate the expression of fibrinogen receptors on platelets of Philadelphia‐negative chronic myeloproliferative neoplasm (MPN) patients. We collected blood samples from 40 consecutive MPN patients and healthy volunteers. We performed flow cytometry analysis of P‐selectin expression and integrin beta‐3, activation of glycoprotein (GP) IIb/IIIa and fibrinogen receptor exposure (PAC‐1 binding). Surprisingly, we found a very low PAC‐1 binding capacity in MPN patients; however, the expression of PAC‐1 was almost completely recovered with aspirin intake. We hypothesize that the hypercoagulable states observed in MPN patients could depend on a primarily plasma‐driven impairment of fibrin turnover and thrombin generation.

Thiol-disulphide Homeostasis in Essential Thrombocythemia Patients

Background

This study aimed to show the status of thioldisulphide homeostasis in essential thrombocytosis patients, which is known to play a role in platelet function.

Methods

The study included 27 ET patients and a control group of 36 healthy subjects. Serum total (-SH + -S-S-) and native (-SH) thiol levels were measured in all subjects using an automatic method.

Results

Age and gender distribution were similar in both groups. Compared with the control group, in the ET group, there were increased native thiol and total thiol levels (p = 0.001, p = 0.046). There was no correlation between thiol, total thiol and disulphide ratios with Jak2 mutation, hemorrhage and thrombosis. A positive correlation was determined between thrombosis and thiol disulphide homeostasis (p = 0.058). The study results showed that thiol-disulphide homeostasis shifted to the proliferative side in ET, in which ineffective erythropoiesis was predominant. It is also known that platelets are more active in ET cases and thiol disulphide balance is important in platelet function.

Conclusions

This result suggests that thrombotic complications may be reduced if the formation is achieved of mechanisms (oxidation mechanisms) that will trigger the increase of disulphide groups. However, more extensive research is needed on this subject.

Acute compartment syndrome as the initial manifestation of chronic-phase chronic myeloid leukemia: a case report and review of the literature

Background

Acute compartment syndrome is an orthopedic emergency requiring urgent fasciotomy to prevent irreversible damage. In hematological malignancies, acute compartment syndrome caused by severe soft tissue bleeding is extremely rare. We present a patient with chronic-phase chronic myeloid leukemia who had acute compartment syndrome caused by severe soft tissue bleeding in her right forearm.

Case presentation

A 72-year-old Japanese woman was referred to our hospital with swelling and pain of her right forearm without a previous history of trauma. She was diagnosed with chronic-phase chronic myeloid leukemia. Extreme thrombocytosis was present, although no evidence of acquired von Willebrand disorder was found. Compartment syndrome caused by soft tissue bleeding was confirmed. An emergency fasciotomy for decompression was conducted. However, sustained postoperative bleeding occurred and required massive red cell concentrate transfusion. As her platelet count decreased by cytoreductive therapy, complete hemostasis was achieved.

Conclusions

Patients with an extremely high platelet count might be at high risk for severe bleeding complications even without acquired von Willebrand disease. For the control of severe bleeding complications in patients with myeloproliferative disorder, the importance of thrombocyte reduction should be recognized.

Bleeding, thrombosis, and anticoagulation in myeloproliferative neoplasms (MPN): analysis from the German SAL-MPN-registry

BACKGROUND

Patients with Ph-negative myeloproliferative neoplasms (MPN), such as polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), are at increased risk for thrombosis/thromboembolism and major bleeding. Due to the morbidity and mortality of these events, antiplatelet and/or anticoagulant agents are commonly employed as primary and/or secondary prophylaxis. On the other hand, disease-related bleeding complications (i.e., from esophageal varices) are common in patients with MPN. This analysis was performed to define the frequency of such events, identify risk factors, and assess antiplatelet/anticoagulant therapy in a cohort of patients with MPN.

METHODS

The MPN registry of the Study Alliance Leukemia is a non-interventional prospective study including adult patients with an MPN according to WHO criteria (2008). For statistical analysis, descriptive methods and tests for significant differences as well as contingency tables were used to identify the odds of potential risk factors for vascular events.

RESULTS

MPN subgroups significantly differed in sex distribution, age at diagnosis, blood counts, LDH levels, JAK2V617F positivity, and spleen size (length). While most thromboembolic events occurred around the time of MPN diagnosis, one third of these events occurred after that date. Splanchnic vein thrombosis was most frequent in post-PV-MF and MPN-U patients. The chance of developing a thromboembolic event was significantly elevated if patients suffered from post-PV-MF (OR 3.43; 95% CI = 1.39-8.48) and splenomegaly (OR 1.76; 95% CI = 1.15-2.71). Significant odds for major bleeding were previous thromboembolic events (OR = 2.71; 95% CI = 1.36-5.40), splenomegaly (OR = 2.22; 95% CI 1.01-4.89), and the administration of heparin (OR = 5.64; 95% CI = 1.84-17.34). Major bleeding episodes were significantly less frequent in ET patients compared to other MPN subgroups.

CONCLUSIONS

Together, this report on an unselected "real-world" cohort of German MPN patients reveals important data on the prevalence, diagnosis, and treatment of thromboembolic and major bleeding complications of MPN.

Haemorrhagic and thrombotic diatheses in mouse models with thrombocytosis

We studied haemostasis in two mouse models with thrombocytosis caused by different pathogenic mechanisms. In one strain (Yall;Mpl-/-) thrombocytosis is driven by a misbalance between thrombopoietin and its receptor, whereas in the other strain, thrombocytosis is caused by expressing a human JAK2-V617F transgene (FF1) that depends on activation by Cre-recombinase (VavCre;FF1, MxCre;FF1). Thrombotic responses were increased following some, but not all types of challenges. In a vaso-occlusive thrombotic model following collagen-adrenaline injection we found increased mortality in both strains. Arterial thrombosis, examined after ferric chloride-induced carotid injury, was accelerated but with little impact on maximal thrombus size. In a vena cava stasis model, clots were of similar size as in wild-type controls, but exhibited a different composition with a higher platelet to fibrin ratio. Both thrombocytosis strains displayed increased haemorrhagic tendency in a tail bleeding assay. Yall;Mpl and VavCre;FF1 displayed a lower proportion of the more reactive high-molecular-weight forms of von Willebrand factor in their plasma, mimicking essential thrombocythaemia with very high platelet counts. Bleeding could not be explained by clear defects in platelet activation, which were normal or only weakly decreased. In conclusion, these models of thrombocytosis recapitulate several features of the haemorrhagic and thrombotic diatheses in ET and PV demonstrating potentials but also some limitations to study these major complications.

Direct evidence for normalization of platelet function resulting from platelet count reduction in essential thrombocythemia

Essential thrombocythemia is characterized by persistent elevation and functional disturbances of platelets. Both the platelet function analyzer-100 (PFA-100) collagen-epinephrine (CEPI) cartridge and aggregometry with epinephrine are considered sensitive and valid methods in detecting abnormal platelet function in essential thrombocythemia. We attempted to confirm that restoration of abnormal platelet function results from platelet count reduction in essential thrombocythemia, by using these two methods. Thirty-nine essential thrombocythemia patients were divided into two groups on the basis of their platelet count. Group A participants (n = 20) exhibited platelet counts greater than 500 × 10/l, whereas group B participants (n = 19) had platelet counts below this limit. Hematological parameters, plasma von Willebrand factor (vWF) antigen and activity levels were assessed. Platelet function was analyzed by the PFA-100 and light transmission aggregometry with epinephrine, collagen, and ADP. The point mutation JAK2 V617F was identified and its effect on platelet function tests was also investigated. By using logistic regression analysis, white blood cell count, vWF activity level, and the measurements of aggregation in response to epinephrine were significantly different between the two groups. Epinephrine-induced aggregation retained the statistical significance in the multivariable procedure (P : 0.002). PFA-100 CEPI closure times were lower - but not statistically significant - in group B. Neither the JAK2 V617F positivity nor different cytoreductive treatments had any influence on ex-vivo platelet function tests. Our findings demonstrate normalization of platelet function resulting from platelet count reduction in essential thrombocythemia and reinforce the concept of lowering platelet counts in these patients.

Essential thrombocythemia

Essential thrombocythemia is a distinct clinical entity within the spectrum of myeloproliferative disorders. There is as yet no pathognomonic diagnostic test, and patients who currently fall into the category of essential thrombocythemia are likely to be heterogeneous. This article discusses diagnostic criteria, clinical features, prognosis, and management.

Thrombocythaemia and pregnancy

The diagnosis of essential thrombocythaemia (ET) is considered when all causes of reactive (secondary) thrombocytosis have been excluded in a patient with a persistent thrombocytosis. Among the chronic myeloproliferative disorders, ET has the highest proportion of young as well as female patients affected. As such, the optimal management of ET during pregnancy becomes an important issue. Although ET is not a contraindication to childbearing, the risks to both the fetus and mother are increased, and would-be-parents should be counselled regarding these. Obstetric complications occurring at higher than expected rates include spontaneous abortion in the first-trimester. Maternal complications, both haemorrhagic and thrombotic, are reported relatively infrequently. It is generally difficult to make specific management recommendations because of the relative rarity of the association between ET and pregnancy, which has precluded adequately powered controlled trials. In this communication, we review our own experience as well as that of other investigators on the subject matter and provide management guidelines that are based on best available information.

What is the standard treatment in essential thrombocythemia

The treatment of patients with essential thrombocythemia (ET) should be based primarily on the expected risk of major thrombotic complications. Although the specific values chosen for separating different risk categories are in part arbitrary, the following recomendations can be made. Young asymptomatic subjects with platelet counts below 1,500x10(9)/L are at lower risk and can be followed untreated. However, it should be emphasized that thrombotic events can also occur in a small percentage of these lower-risk cases. Low-dose aspirin (100-300 mg/day) should be given to patients with symptoms of microvascular occlusion, such as erythromelalgia or transient neurological attacks, and avoided in those with bleeding manifestations. The risk/benefit of low-dose aspirin in the primary prevention of thrombosis in asymptomatic patients remains uncertain. For high-risk patients (age >60 years, or platelet count >1,500x10(9)/L, or previous thrombosis), hydroxyurea, plus aspirin in the case of thrombosis, is the treatment of choice because its efficacy in preventing thrombotic complications has been proven in a randomized clinical trial. However, the possible long-term leukemogenicity of this drug, as well as that of other effective cytoreductive agents such as busulphan and pipobroman, remains a major concern. Anagrelide and interferon could overcome this worry but their efficacy has been hitherto demonstrated only in lowering the platelet count. Controlled clinical studies showing a benefit in preventing thrombotic and hemorrhagic complications are urgently needed.

Polycythemia vera presenting as complex regional pain syndrome of the lower limbs

CASE REPORT

A 73-year-old woman with bilateral severe pain in her legs and feet was admitted to the hospital. The pain had started spontaneously without any known trauma a year earlier and had progressively worsened to the point that she was unable to walk. The patient complained of pain, discoloration, and sweating of the lower extremities. Physical examination revealed swollen, cold, wet, and blue extremities, with both allodynia and hyperalgesia, fulfilling criteria for a diagnosis of complex regional pain syndrome (CRPS) type 1. Laboratory findings demonstrated a hemoglobin level of 180 g/L(-1) and a hematocrit of 47%, leading to a diagnosis of polycythemia vera (PCV). Her condition slowly improved after repeated venesection, mild analgesics, and amitriptyline.

CONCLUSIONS

Polycythemia vera has not been described previously as a contributing factor in CRPS. In the present case, the diagnosis of PCV was important, because its treatment also improved the CRPS.

Increased circulating platelet-leukocyte aggregates in myeloproliferative disorders is correlated to previous thrombosis, platelet activation and platelet count

Platelet-leukocyte adhesion may occur as a consequence of platelet activation and possibly plays a key role in the deposition of activated platelets and fibrin in the thrombotic plug. The aim of the present study was to assess by whole blood flow cytometry the presence of circulating platelet-leukocyte aggregates (PLA) and the platelet-leukocyte response to platelet agonist stimulation (ADP and TRAP) in 50 patients with chronic myeloproliferative disorders (MPD) and 30 controls. PLA were identified as platelet-granulocyte/monocyte aggregates (PGMA), platelet-monocyte aggregates (PMA) and defined as the percentage of leukocytes coexpressing the platelet-specific marker glycoprotein Ib. Compared to controls the mean percentage of PGMA and PMA was increased in unstimulated whole blood from patients with MPD (7.98 vs. 1.76%; p<0.001 and 12.34 vs. 3.2%; p<0.001, respectively). The percentage of PGMA was correlated to the platelet count (r=0.46; p<0.001), percentage of P-selectin (r=0.69; p<0.001) and thrombospondin (r=0.58; p<0.001) positive platelets and platelet expression of GPIV (r=0.33; p=0.02). The mean percentage of PGMA and PMA was significantly increased in ADP-stimulated whole blood of patients (57.14 vs. 47.92%; p=0.009 and 54.91 vs. 45.89%; p<0.001, respectively). Compared to patients without a history of thrombosis, patients having experienced microvascular disturbances or a thrombotic event had a higher mean percentage of PGMA and PMA in non-stimulated whole blood (10.07 vs. 6.34%; p=0.025 and 14.81 vs. 10.48%; p=0.021, respectively) and a higher percentage of PGMA in ADP stimulated whole blood (64.32 vs. 51.50%; p<0.01). These data document an increased frequency of PLA in non-stimulated whole blood in MPD associated with a previous history of thrombosis or microvascular disturbances.

Hemostatic Changes in Patients with Malignancy

Alterations of hemostasis commonly accompany the progression of malignant disease and every known component of the hemostatic mechanism may be affected by this disease process. Nearly all patients with an active neoplasm will exhibit at least subtle biochemical changes in hemostasis, and a minority of these patients will also develop clinical thrombosis or hemorrhage. In this paper, we will review intravascular coagulation and fibrinolysis, thrombocytopenia, and thrombocytosis, as well as more rare thrombotic and hemorrhagic events resulting from the direct interactions of neoplasms, or of their products, with the individual elements of hemostatic mechanisms. Thrombotic and hemorrhagic events resulting from the induction of autoimmune or thrombotic microangiopathic syndromes are also discussed. This review focuses on the clinical thrombotic and bleeding syndromes that may occur as a result of this interaction between neoplasia and hemostasis.

Effectiveness of anagrelide in the treatment of symptomatic patients with essential thrombocythemia

We prospectively evaluated the effect of anagrelide on platelet counts and the clinical manifestations of microvascular circulation disturbances in 17 newly diagnosed patients with essential thrombocythemia. Ten patients had symptoms related to thrombocythemia, eight at the time of starting anagrelide treatment. The platelet counts before anagrelide treatment and during maintained remission of essential thrombocythemia by anagrelide were 980 (range, 610-2030) and 378 (range, 212-546) x 10(9)/L, respectively. Spontaneous platelet aggregation was found in 6 patients (35%), which disappeared on remission of essential thrombocythemia in five cases (P = 0.02). Essential thrombocythemia-related microvascular thrombotic and hemorrhagic symptoms disappeared with the normalization of platelet count in all cases during maintained remission of essential thrombocythemia by long term continuous anagrelide treatment with a follow-up period of between 2 and 6 years. However, ET-related symptoms reappeared in three patients, coinciding with increased platelet count up to 600 x 10(9)/L caused by anagrelide dose reduction. We conclude that reduction of increased platelet to normal (< 400 x 10(9)/L) in symptomatic patients with essential thrombocythemia through use of maintained anagrelide treatment is associated with the disappearance of spontaneous platelet aggregation and the complete relief of thrombotic and hemorrhagic manifestations.

Increased platelet activation and abnormal membrane glycoprotein content and redistribution in myeloproliferative disorders

Chronic myeloproliferative disorders (MPDs) are characterized by a high incidence of thrombohaemorrhagic complications, possibly caused by platelet dysfunction. In an attempt to define platelet functional abnormalities, we assessed the expression of activation-dependent membrane proteins in unstimulated and agonist [ADP and thrombin receptor-activating peptide (TRAP)]-stimulated platelets using quantitative whole blood flow cytometry in samples from 50 MPD patients and 30 controls. The receptor densities of activation markers and glycoproteins (GPs) were quantified using standardized fluorescent beads. Compared with controls, the mean percentage of P-selectin-positive (15.3% vs. 7.2%; P < 0.001) and thrombospondin (TSP)-positive (6.6% vs. 3.7%; P = 0.003) platelets was increased in unstimulated platelets from patients. Patients having experienced a thrombotic event had a higher mean percentage of TSP-positive non-stimulated platelets than patients without a history of thrombosis (9.0% vs. 4.6%; P = 0.02) and a higher GPIV molecules of equivalent fluorochrome (MEF) value (33113 vs. 24471 MEF; P = 0.02). Mean MEF values of monoclonal antibodies (mAbs) against GPIb (34055 vs. 38945 MEF; P < 0.001) and GPIIb/IIIa (1416 vs. 1648 MEF; P < 0. 001) were significantly reduced among patients, whereas surface expression of GPIV was increased in patients (28273 vs. 16258 MEF; P < 0.001). In TRAP (10 micromol/l) stimulated whole blood, the MEF of P-selectin (9611 vs. 13293 MEF; P = 0.004) and CD63 (2385 vs. 5177 MEF; P < 0.001) and the ratio of PAC-1/GPIIb/IIIa MEF (0.98 vs. 2. 00; P < 0.001) was reduced in patients, indicating either a reduced granule GP content or an intrinsic cellular defect in receptor-mediated granule secretion and activation of the GPIIb/IIIa complex. Expressed as the relative change of MEF compared with unstimulated platelets, TRAP induced decrease of GPIb (7.8% vs. 45%; P < 0.001) and increase of GPIIb/IIIa (49.1% vs. 95.7%; P < 0.001) and GPIV expression (17.8% vs. 55.2%; P < 0.001) was attenuated in patients.

Increased platelet surface expression of P-selectin and thrombospondin as markers of platelet activation in essential thrombocythaemia

Essential thrombocythaemia (ET) is a clonal myeloproliferative disorder associated with an increased risk of both thromboembolic and bleeding complications. Platelet activation plays a crucial role in the pathogenesis of prethrombotic conditions. The platelet surface expression of p-selectin (CD62p) and thrombospondin (TSP) has been shown to correlate with platelet activation. In the present study, we used a flow cytometric assay to study whether the fraction of platelets expressing CD62p and TSP is increased in newly diagnosed ET. Thirty-four patients with newly diagnosed ET and 25 healthy control subjects were investigated. The proportion of platelets expressing the activation-dependent antigens CD62p and TSP was higher in patients with ET (CD62p: 14.7+/-15.0%; TSP: 12.4+/-9.9%) as compared with healthy control subjects (CD62p: 3.0+/-4.0%; TSP: 3.2+/-3.2%; p< 0.001). In ET, there was a linear correlation between platelet surface expression of CD62p and TSP (p<0.0001, r=0.83). At diagnosis of ET, 20 patients were symptomatic and 14 asymptomatic. Compared with asymptomatic ET patients there was no difference in the expression of CD62p (18.3+/-16.2% vs. 14.5+/-13.4%) and TSP (14.4+/-9.8% vs. 12.8+/-9.5%) in symptomatic ET patients. In conclusion, increased expression of platelet neoantigens is present at the diagnosis of ET. Both activation-dependent epitopes CD62p and TSP are increasingly expressed on the platelet surface in newly diagnosed ET patients.

Clinicopathological diagnosis and differential criteria of thrombocythemias in various myeloproliferative disorders by histopathology, histochemistry and immunostaining from bone marrow biopsies

Differentiation of essential thrombocythemia (ET) from thrombocythemias occurring in various subtypes of chronic myeloproliferative disorders (MPDs) is controversial, because of the lack of uniform clinical and morphological criteria. A retrospective clinicopathologic study was performed on 375 patients presenting with a MPD and a platelet count exceeding 500 x 10(9/)l. For comparison 35 patients with reactive thrombocytosis (RT) and five patients with a myelodysplastic syndrome (MDS-5q(-) syndrome) were enrolled into this study. In addition to a complete clinicopathological work-up, procedures included histochemical and immunological staining techniques and morphometry of bone marrow biopsies for proper evaluation of megakaryocytes (CD61) and erythroid precursors (Ret40f). Because of the high patient's age on admission, relative survival rates with corresponding disease-specific loss of life expectancy were calculated. Analysis of clinical and morphological characteristics, in particular megakaryopoiesis revealed features which enabled a clear-cut distinction between thrombocythemias in MPDs and thrombocythemic states in MDS. This rationale proved to be most important for the diagnostic discrimination of the 33 patients with initial (prefibrotic) stages of idiopathic myelofibrosis (IMF) from ET (40 patients). A new set of relevant criteria for the diagnosis of IMF with special regard to early stages and its distinction from ET has been proposed. Hemorrhagic episodes were more frequently observed in ET than in thrombocythemias associated with polycythemia vera (PV). Computation of specific loss of life expectancy revealed two extremes: thrombocythemia in CML (81%) and ET (3%), whereas thrombocythemias in PV and IMF did not show a significantly different life loss (19-22%). The revised criteria for ET, PV and IMF are reliable by taking histopathological features from bone marrow biopsies into consideration, particularly for the diagnosis of ET and its differentiation from thrombocythemias as a presenting symptom accompanying the various subtypes of MPDs.

Studies of platelet volume, chemistry and function in patients with essential thrombocythaemia treated with Anagrelide

Anagrelide (imidazoquinazolin derivative) is a new compound proposed for the treatment of myeloproliferative disorders. In this study, Anagrelide was given to patients with essential thrombocythaemia (ET) in a compassionate-use protocol. The aim of this study was to test the effect of this drug not only on the platelet count but also on platelet volume, chemistry and function, which has not previously been reported. Thus, in ET, different functional or structural platelet abnormalities were reported: a shortening of the bleeding time, hypoaggregation to several agonists, and in particular a lack of response to adrenalin, an increase in the amount of total platelet glycoprotein IV (or CD36), and an abnormal migration of thrombospondin on electrophoresis. These different parameters were studied before and during therapy with Anagrelide. Although the platelet count was corrected, no functional or chemical abnormality was improved. Furthermore, platelet volume was shown to be constantly increased under Anagrelide. Thus, Anagrelide, in reducing the platelet count, may possibly decrease the risk of thrombosis and haemorrhage. Nevertheless, if the risk of thromboses and/or myelofibrosis is related not only to the platelet count but also to the platelet abnormalities, the persistence of a thrombocytopathy in patients treated with Anagrelide must be taken in consideration. Our data suggest that thromboses and myelofibrosis are clinical end-points which should be included in future large-scale use of Anagrelide.

The use of aspirin in polycythaemia vera and primary thrombocythaemia

In polycythaemia vera (PV; polycythaemia rubra vera, primary proliferative polycythaemia) and primary thrombocythaemia (PT; essential thrombocythaemia), occlusive complications in the microvasculature and larger vessels are a significant cause of morbidity and mortality. Central to the pathogenesis of these complications are the quantitative and qualitative platelet changes present in these myeloproliferative disorders. Aspirin irreversibly inactivates cyclo-oxygenase in platelets. This leads to a reduced production of platelet thromboxane A2 which has vasoconstricting and platelet aggregatory properties. In haematologically normal individuals, aspirin has been shown to reduce thrombo-embolic complications in populations at risk of these events. In PV and PT, aspirin has been shown to specifically eliminate the micro-circulatory and vasomotor manifestations and there is some evidence of a reduction in larger vessel occlusion. Low-dose aspirin has been shown to substantially reduce the raised thromboxane A2 production of platelets in PV and PT patients. The incidence of haemorrhagic side-effects of aspirin are minimized by the use of low doses. Haemorrhagic events are particularly found in patients with platelet counts > 1000 x 10(9)/l and these events are enhanced by aspirin therapy in these patients. Aspirin should be used with caution in patients with dyspeptic symptoms or a history of peptic ulceration or bronchospasm. Precise PCV control (< 0.45) and cytoreduction (platelets < 400 x 10(9)/l) should be used in patients with PV to minimize the vascular occlusion risk but routine cytoreduction is proposed only for those at particular risk of vascular occlusion in PT. In the acute presentation of patients with vascular occlusion, cytoreduction and an aspirin dose of 300 mg a day is proposed, reducing to 75 mg a day with the control of symptoms and signs, while 75 mg a day may play a role as prophylactic therapy in the prevention of thrombosis. However, there are no prospective studies in PT to demonstrate the benefit/risk profile and to confirm these recommendations, while a randomized prospective placebo-controlled study of low-dose aspirin in PV has only recently been initiated.