Platelet aggregation disorders

Irrespective of their mechanism of action, anticoagulants reduce the formation and action of thrombin. Thus they interfere with a final step in coagulation. Among platelet inhibitors only the GPIIb/IIIa antagonists inhibit the common pathway of aggregation, namely the formation of platelet-to-platelet bridges which are mediated by fibrinogen or von Willebrand factor. In contrast, acetylsalicylic acid (ASA), NSAIDs, clopidogrel (Plavix) or ticlopidine (Tyklid) inhibit platelet activation by abrogating the formation or action of a secondary platelet agonist, namely of thromboxane A(2) or ADP. They do not block platelet aggregation which is directly induced by thrombin. Therefore, ASA, clopidogrel, or ticlopidine are not associated with a significant risk of bleeding as long as other factors such as an extensive thrombocytopenia or anticoagulation are not involved. Therefore, in contrast to anticoagulants, therapeutic drug monitoring is not necessary with ASA, clopidogrel, nor ticlopidine. On the other hand, ASA has even to be applied at a dosage that almost completely inhibits thromboxane synthesis in order to act on platelet aggregation at all. Among the GPIIb/IIIa-antagonists only parenteral drugs have been approved for therapeutic use, e. g. abciximab (ReoPro), eptifibatide (Integrilin), and tirofiban (Aggrastat). Clinical studies revealed an increased risk of bleeding without a sufficient therapeutic benefit of oral GPIIb/IIIa antagonists. GPIIb/IIIa antagonists may induce thrombocytopenia that is attributed to an out-side-in signalling or immunological phenomena. A test system (Ultegra, Accumetrics) is available for a therapeutic drug monitoring of GPIIb/IIIa antagonists. However, estimation of the bleeding risk always requires an evaluation of all factors influencing the haemostatic system, especially when heparin or other inhibitors are applied additionally.     

Platelet abnormalities in myeloproliferative disorders

A large number of various platelet abnormalities are described in patients with MPD. These abnormalities serve diagnostic purposes only. They appear to have little or no predictive value regarding the clinical manifestations of the patients or the progress of the disease. Those platelet characteristics most consistently reported to be defective include a decrease in the platelet content of serotonin and adenine nucleotides, decreased platelet density, an abnormal ultrastructure characterized by paucity of granules and hypertrophy of the surface connecting canicular system, an altered membrane glycoprotein profile that includes reduced levels of GPIb, and reduced lipoxygenase activity and aggregation response with epinephrine. These abnormalities may originate at the megakaryocyte level. Furthermore, the released abnormal platelets may undergo modification of their functional and biochemical characteristics as a result of episodes of intravascular thrombosis or aging in circulation or as a result of the progression and treatment of the disease, thus creating the paradoxic and often conflicting relationship between the thrombotic and hemorrhagic events and the results of platelet functional tests as observed in this disorder.     

Platelet alpha-granule release in chronic myeloproliferative disorders with thrombocytosis

Platelet alpha-granule release in 22 patients affected by chronic myeloproliferative disorders with thrombocytosis and seven subjects with thrombocytosis secondary to splenectomy were studied. We found elevated beta-thromboglobulin (BTG) and platelet factor 4 (PF4) plasma levels in all patients. Intraplatelet content of BTG and PF4 was decreased in patients with idiopathic thrombocythaemia (IT) and idiopathic myelofibrosis (IMF). The BTG and PF4 results were also expressed as the ratio plasma BTG and PF4: whole blood platelet count. In patients with IT, BTG: whole blood platelet count ratio was low, conversely, the same ratio was high in patients with IMF. In conclusion, our results suggest the presence of an abnormal, BTG-deficient clone in IT and a peripheral platelet activation in IMF.     

Platelet survival, platelet factor-4 and bleeding time in myeloproliferative disorders

In 27 patients with myeloproliferative disorders observations on thrombohaemorrhagic complications, platelet function tests and spleen size were made. Sixteen patients had thrombotic or haemorrhagic episodes. All 27 patients had elevated platelet factor-4 and 25 patients had a shortened platelet survival. Patients with myelofibrosis had a significantly shorter platelet survival than patients with polycythaemia vera (p less than 0.05). Seven out of 23 patients investigated had prolonged bleeding time. The observed abnormalities of platelet function tests were not related to thrombohaemorrhagic complications or spleen size.     

Platelet count

An estimate of the platelet count is usually given as part of the complete blood count (CBC). It should not be neglected when reviewing CBC results as it is a fair screening test and satisfactory for most purposes in the emergency department. It is a quantitative study, and the functional status of the platelet must always be considered in interpretation of clinical significance. Patients presenting with abnormal bleeding histories, particularly women with mucosal origin bleeding, should have an evaluation of platelet quantity by an accurate method and functional quality by a bleeding time. Though rarely used, the emergency physician should be familiar with the indication and mechanics of platelet transfusions. The laboratory charge for the platelet count is $17.00 to 18.00, whether it is automated or hand counted.     

Platelet thromboembolism

Abstract. Platelet-dependent thrombosis and subsequent embolization are major causes of cerebral ischaemia. Beside aspirin which irreversibly blocks platelet cyclo-oxygenase, several other substances interfere in different platelet metabolic pathways and block platelet adhesion and aggregation. We found in an experimental model using non-human primates that a specific peptide inhibitor blocking GP IIb/IIIa platelet receptor which binds fibrinogen completely, prevents the retention of embolized platelet aggregates in the cerebral circulation. As thrombin may play a key role for platelet activation in vivo leech-derived hirudin, a direct thrombin inhibitor as well as activated protein C which limits thrombin production and also prevents platelet dependent thrombus formation very effective. We demonstrated in the same non-human primate model of platelet embolization that the amount of retention of platelet emboli in the vascular bed depends on the nature of the vasculature. For example, platelet emboli were cleared very quickly from brain microcirculation, whereas platelet embolization into the lower limb via the femoral artery caused a significantly longer retention of the embolized material. Such specific mechanisms may be caused by different levels of local vasodilators as PGI₂ or EDRF.     

Platelet Disorders

Practical considerations concerning long-term oral anticoagulant therapy are often the responsibility of the primary care physician. Decisions need to be made regarding dose adjustment, frequency of follow-up, and interruption of therapy for dental and noncardiac surgical procedures. Drs Sebastian and Varkey address these and other aspects of anticoagulant therapy and also review indications, contraindications, and complications.     

Platelet pharmacogenomics

Summary. Platelet responsiveness to conventional antiplatelet therapy underlies a high interindividual variability influenced by various factors. For instance, antiplatelet therapy does not curtail the expected effects in a relevant number of patients as demonstrated by the occurrence of repeated cardiovascular events including stent thrombosis and/or by inadequate platelet inhibition measured by in vitro platelet function assays. Besides non‐genetic factors such as age, gender, liver and renal function and co‐medication, considerable variation of antiplatelet drug responsiveness can be attributed to genetic factors including polymorphisms and genetic variants of platelet surface proteins and drug metabolizing enzymes. Nowadays, platelet pharmacogenomics has started a new field with the goal to link genetic information of various drug targets to interindividual variability of drug response. Evolving data from large cohort studies suggest a promising role for pharmacogenomics in the context of antiplatelet therapy. Additionally, with the revolution of low cost and high‐throughput genotyping techniques, genetic testing has become affordable for clinical application and individualization of therapy. However, a key issue to define the future role of pharmacogenomics will rely on the benefit and the timeliness of implementing the genetic information into therapeutic decision. Hence, it warrants further investigations to document the prognostic effects of therapeutic alterations in distinct genotypes. Concerning the safety profile of emerging antiplatelet and antithrombotic drugs in certain risk groups it would be fatal to individualize treatment barely on behalf of an atherothrombotic genotype. In contrast, individual risk assessment combining non‐genetic information and pharmacogenetic analysis represents a reasonable concept. Here, we provide a review on current data describing the role of pharmacogenomics in the field of antiplatelet drug treatment in cardiovascular patients with future directions.     

Platelet proteomics

Summary.  As anucleate cell particles, platelets do not lend themselves to analysis by traditional cell and molecular biology techniques. Moreover, while valuable information may be gleaned from studies of messenger RNA in platelets, the rapid events in platelets are not governed by or dependent on alterations in gene expression. In contrast, proteomics, the study of the protein complement of a genome, will have a major impact on platelet biology. It offers the opportunity to comprehensively describe the proteins involved in discrete elements of platelet function, from the subsecond events following platelet activation and adhesion through to platelet aggregation and granule secretion. As the function of every protein is understood and as the mechanisms that regulate protein modifications are unravelled, we will discover a wealth of proteins that are themselves potential therapeutic agents or novel targets for the development of diagnostics and drugs. Here we review the current applications of proteomics to platelet research. We briefly describe various proteomic approaches to unravel platelet biology, including the documentation of platelet proteins, the investigation of thrombin-activated phosphotyrosine signaling networks and the analysis of the proteins that are secreted upon platelet activation. Proteomics is a young field and there are only a handful of published examples applying proteomics to platelet research. This number will increase over the next few years, as advances in analytical methods allow a more functional analysis of the platelet proteome.     

EDTA依赖性血小板聚集及其对血小板计数的影响

目的 探讨动态分析EDTA-K2依赖性血小板聚集现象随时间的变化.方法 随机选取内科门、急诊无血小板聚集的正常EDTA-K2抗凝血常规标本10份(血小板数量大于10×109/L且小于600×109/L的标本5份,小于10×109/L的5份)和出现明显EDTA-K2依赖性血小板聚集的血常规标本10份,其中每份聚集标本备有枸橼酸钠抗凝血标本1份(同一患者).将各标本于同一时间内,在同等条件下用3种方法(电阻抗法、激光散射法、手工法)进行血小板计数,记录各测定值,用Graphpad prism5软件进行线性回归及相关性分析;在标本采集后不同时间点(30min,60 min,90 min),用血球仪对聚集标本进行血小板计数,同时涂片瑞士染色观察,动态分析血小板聚集随时间的变化情况.结果 无血小板聚集组,3种方法测定值间均具有良好的相关性,相对偏差为临床可接受;血小板聚集现象组,3种方法测定值间相关性较差,相对偏差超出临床可接受范围;当用相应的枸橼酸钠抗凝标本检测时,发现各测定值间相关性较好,其相对偏差亦为临床可接受;此外,随时间推移,EDTA-K2依赖性血小板聚集会逐渐增强.结论 血小板聚集会影响血小板检测,使得各方法测定值间失去可比性与相关性,其相对偏差超出临床质控可接受范围,同时血小板聚集现象会随着时间推移更加明显,故一旦发现血小板聚集标本,应尽快进行人工计数,并通知临床重新抽取枸橼酸钠抗凝标本复检.