FIGURE 16-1. Relationship between platelets and the clotting cascade in the generation of a stabilized fibrin clot. 5HT = serotonin; ADP = adenosine diphosphate; PF4 = platelet factor 4; TxA 2 = thromboxane A 2.
Once adhesion occurs, platelets change shape and activation occurs. Substances such as collagen, ADP, thrombin, and thromboxane A 2 (TxA 2) stimulate the change in platelet shape and cause platelets to release their contents including ADP, serotonin (5HT), platelet factor 3 (PF3), and platelet factor 4 (PF4).2 Following platelet adhesion and activation, platelet aggregation completes the formation of the hemostatic plug. This process is mediated by glycoprotein IIb/IIIa receptors on the platelet surface with fibrinogen acting as the primary binding ligand bridging between platelets. Platelets have numerous Gp IIb/IIIa binding sites, which are an attractive option for antiplatelet drug therapy.2,4
ADP and TxA 2 recruit additional platelets, which aggregate to the platelets that are already bound to the subendothelial tissues. In addition to promoting aggregation, TxA 2, 5HT, and other substances are potent vasoconstrictors that limit blood flow to the damaged site. When vascular damage is minimal, the vasoconstriction and platelet aggregation (formation of a platelet plug) may be sufficient to limit bleeding.
However, the platelet plug is not stable and can be dislodged. To form a more permanent hemostatic plug, the clotting system must be stimulated. By releasing PF3, platelets initiate the clotting cascade and concentrate activated clotting factors at the site of vascular (endothelial) injury.
Prostaglandins (PGs) play an important role in platelet function. Figure 16-2 displays a simplified version of the complex arachidonic acid pathways that occur in platelets and on the vascular endothelium. Thromboxane A 2, a potent stimulator of platelet aggregation and vasoconstriction, is formed in platelets. In contrast, prostacyclin (PG2), produced by endothelial cells lining the vessel luminal surface, is a potent inhibitor of platelet aggregation and a potent vasodilator that limits excessive platelet aggregation.
FIGURE 16-2. Formation of thromboxane A 2 (TxA 2), prostaglandins (PGs), and prostacyclin in platelets and vascular endothelial cells. CO = cyclooxygenase; ASA* = low-dose, irreversible, inactivation of platelet cyclooxygenase; ASA/ = high-dose inactivation of platelet cyclooxygenase.
Cyclooxygenase and PG2 are clinically important. An aspirin dose of 80 mg/day acetylates and irreversibly inhibits cyclooxygenase in the platelet. Platelets are rendered incapable of forming arachidonic acid and PGs. This effect of low-dose aspirin lasts for the lifespan of the exposed platelets (up to 12 days).
Vascular endothelial cells also contain cyclooxygenase, which converts arachidonic acid to PG2. Aspirin in high doses inhibits the production of PG2. However, because the vascular endothelium can regenerate PG2, aspirin’s effect is much shorter here than on platelets. Thus, aspirin’s effect at high doses may both inhibit platelet aggregation and block the aggregation inhibitor PG2. This phenomenon is the rationale for using low doses of aspirin 80–325 mg/day to help prevent myocardial infarction.
In summary, a complex interaction between the platelet and blood vessel wall maintains hemostasis. Once platelet adhesion occurs, the clotting cascade may become activated. After thrombin and fibrin are generated, the platelet plug becomes stabilized with insoluble fibrin at the site of vascular injury.
The ultimate goal of the coagulation cascade (Figure 16-3) is to generate fibrin from thrombin. Fibrin forms an insoluble mesh surrounding the platelet plug. Platelets concentrate activated clotting factors at the site of vascular injury.
FIGURE 16-3. Coagulation cascade. Dotted lines indicate thrombin’s feedback action, which modifies factors V and VIII. HMWK = high molecular weight kininogen. (Reproduced, with permission, from reference 5.)
The nomenclature and half-lives for the coagulation proteins are shown in Table 16-1. The coagulation cascade is typically divided into the intrinsic, extrinsic, and common pathways: the intrinsic and extrinsic pathways provide different routes to generate factor X while the common pathway results in thrombin formation. Coagulation is initiated by vascular injury or damage that exposes blood to tissue factor (TF), which then binds to factor VII at the start of the extrinsic pathway. The binding of TF to factor VII activates the latter to VIIa. The complex formed by TF and factor VIIa can then activate factor X to Xa at the start of the common pathway. Alternatively, the TF-factor VIIa complex can first convert factor IX to factor IXa, with factor VIIIa as a cofactor, which is part of the intrinsic pathway. Factor IXa can then activate factor X into Xa; thus, both the intrinsic and extrinsic pathways activate factor X in the final common pathway. Factor Xa with factor Va as a cofactor activates prothrombin (factor II) into thrombin (factor IIa). In the clotting cascade, thrombin not only converts fibrinogen into fibrin, but it can also convert factor XIII to factor XIIIa, which stabilizes the fibrin clot.4 In addition to the direct effects and feedback mechanisms of thrombin shown in Figure 16-3, thrombin also stimulates platelet aggregation and activates the fibrinolytic system.
TABLE 16-1. Characteristics of Coagulation Factors 1,6
Factors such as calcium and vitamin K play an intricate role within the various pathways in the coagulation cascade. Calcium is essential for the platelet surface binding of several factors within the pathway. Vitamin K facilitates the calcium binding function of factors II, VII, IX, and X via carboxylation. These processes are critical in activating proteins within the pathway.
Mechanisms that limit coagulation include the natural inhibitors such as antithrombin (AT) and the vitamin K dependent proteins C and S, tissue factor pathway inhibitor (TFPI), and the fibrinolytic system. Endothelial cells produce several substances that have antithrombotic effects which may also activate the fibrinolytic system.4 Platelet aggregation is prevented by substances such as PG2 and nitric oxide, both of which are generated by the vessel wall.1,4 Generation of plasminogen activators also can limit platelet aggregation.1 Several medications can also inhibit coagulation by acting on platelets (aspirin, clopidogrel, prasugrel) or one or more clotting factors (warfarin, low molecular weight heparins [LMWHs], unfractionated heparin [UFH], dabigatran, rivaroxaban, apixaban, and direct thrombin inhibitors [DTIs]).
High concentrations of thrombin, in conjunction with thrombomodulin, activate protein C, which can then inactivate cofactors Va and VIIIa; thus, there is a negative feedback mechanism that will block further thrombin generation and subsequent steps in the coagulation cascade.8 Protein S serves as a cofactor for protein C. Antithrombin inactivates thrombin as well as factors IX, X, and XI, and this process can be hastened by heparin. Heparin and AT combine one-to-one, and the complex neutralizes the activated clotting factors and inhibits the coagulation cascade. Deficiencies in these natural inhibitors can result in increased generation of thrombin, which can lead to recurrent thromboembolic events often starting at a young age. Tissue factor pathway inhibitor impedes the binding of TF to factor VII, essentially inhibiting the extrinsic pathway (Figure 16-3). Unfractionated heparin and LMWHs can release TFPI from lipoproteins.4 The complex mechanisms that limit thrombus formation are shown in Figure 16-4.
FIGURE 16-4. Exogenous, extrinsic, and intrinsic pathways for activation of plasminogen. APSAC = anistreplase; SK = streptokinase; SK/PG = streptokinase-plasminogen complex; UK = urokinase. (Reproduced, with permission, from reference 7.)
Fibrinolysis is the mechanism by which formed thrombi are lysed to prevent excessive clot formation and vascular occlusion. As discussed previously, fibrin is formed in the final common pathway of the clotting cascade. Tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) activate plasminogen, which generates plasmin. Plasmin is the enzyme that eventually breaks down fibrin into fibrin degradation products (FDPs). Medications can either activate (e.g., streptokinase, alteplase, urokinase, reteplase, and tenecteplase) or inhibit (tranexamic acid, aminocaproic acid, and aprotinin) fibrinolysis.
For the purpose of discussion, bleeding and clotting disorders are organized by tests that assess
Tests to assess platelets include platelet count, volume (mean platelet volume [MPV]), function (e.g., bleeding time [BT] and platelet aggregation), and others. Thrombin time (TT), reptilase time, prothrombin time (PT)/international normalized ratio (INR), activated partial thromboplastin time (aPTT), activated clotting time (ACT), fibrinogen assay, and others are laboratory tests that assess coagulation. Clot degradation is assessed with tests for FDPs and D-dimer. Fibrinolysis also is monitored with the euglobulin lysis test.
A hypercoagulable state workup may include activated protein C (APC) resistance and the factor V Leiden mutation, anticardiolipin antibody, antiphospholipid antibody, antiplasmin, AT, C-reactive protein, heparin neutralization, homocystine, lipoprotein, plasminogen, plasminogen activator inhibitor 1 (PAI1), platelet hyperaggregation, proteins C and S, prothrombin G20210A mutation, reptilase time, and TT. These tests are often performed in panels since the presence of more than one predisposition to thrombosis further increases the risk for thrombosis.8
In addition, general hematological values such as hemoglobin (Hgb), hematocrit (Hct), red blood cell (RBC) count, and white blood cell (WBC) count, as well as urinalysis and stool guaiac tests may be important to obtain when evaluating blood and coagulation disorders; some of these tests are further discussed in Chapter 15: Hematology: Red and White Blood Cell Tests. Table 16-2 is a summary of common tests used to evaluate bleeding disorders and monitor anticoagulant therapy.
TABLE 16-2. Summary of Coagulation Tests for Hemorrhagic Disorders and Anticoagulant Drug Monitoring a
aPTT= activated partial thromboplastin time; BT= bleeding time; ITP= idiopathic thrombocytopenic purpura; PT/INR= prothrombin time/international normalized ratio; TTP= thrombotic thrombocytopenic purpura; vWF= von Willebrand factor; WNL= within normal limits.
a Italic type indicates most useful diagnostic or therapeutic tests.
b Significant thrombocytopenia may occur as a heparin side effect in 1% to 5% of patients.
Normal range: 150,000–450,000/µL
The only test to determine the number or concentration of platelets in a blood sample is the platelet count, through either manual (rarely done) or automated methods. Interferences with platelet counts include RBC fragments, platelet clumping, and platelet satellitism (platelet adherence to WBCs). Automated platelet counts are performed on anticoagulated whole blood. Most instrumentation that performs hematological profiles provides platelet counts. Platelets and RBCs are passed through an aperture generating an electric pulse with a magnitude related to the size of the cell/particle. The pulses are counted, and the platelets are separated from the RBCs by size providing the platelet count and MPV as well as the RBC count and mean corpuscular volume (MCV).
An abnormal plat
