In the contemporary view of haemostasis, clotting includes three distinct phases: initiation, amplification and propagation (Figure 1)1, 2.
- Initiation involves the exposure of tissue factor to coagulation factors, with subsequent activation of other factors and the generation of a small amount of thrombin, which converts fibrinogen to fibrin.
- In the amplification phase, coagulation factor production is increased and thrombin production is accelerated. This also promotes maximal platelet activation1, 2.
- Finally, during propagation, activated platelets are recruited to the site of injury and there is a burst of thrombin production. This leads to the conversion of fibrinogen to fibrin and the production of a stable fibrin clot1-3.
Figure 1: The three stages of coagulation.2
Platelets are small, flat, disc-shaped blood cells that circulate in the blood and help to form blood clots.4 In the event of vascular injury, platelets are activated by adhesion to von Willebrand factor or collagen, and by circulating soluble platelet-activating agonists. The activation causes them to change shape, release further platelet activating agonists, including ADP and thromboxane A2,5,6 and aggregate to form a primary platelet plug (primary haemostasis).1 Further platelets are activated by these circulating agonists, or by fibrinogen binding to the GpIIb/IIIa receptors. Fibrin monomers polymerise into a fibrous gel,4 and stabilise and grow within this mesh of fibrin and aggregated platelets, forming a clot (secondary haemostasis).1
Fibrinolysis is the process of removing a fibrin clot once the injured blood vessel has healed.7 The principal mediator of this is plasmin, which breaks down the fibrin polymer to produce soluble fibrin degradation products.2,7–9Plasmin is produced from circulating plasminogen, which is synthesised in the liver.2,8,9 The conversion of plasminogen to plasmin is facilitated by two types of plasminogen activators, released from endothelial cells (Figure 2).2,8
As the presence of plasmin increases plasminogen activator activity, plasminogen therefore exerts positive feedback on its own activation and ensures the fibrin clot is removed appropriately.7 To avoid excessive degradation of fibrin and to maintain balanced haemostasis, plasminogen activator inhibitors also regulate this process by forming inactivating complexes with the plasminogen activators to prevent inappropriate plasmin generation.2,10
Figure 2. Overview of the fibrinolytic system.9
Laboratory tests and assessments are able to specifically identify the levels of individual components of the coagulation pathway, which can help diagnose the cause of patient bleeding or clotting (Table 1). They use plasma samples, as opposed to whole blood, with levels of coagulation components assessed separately through individual tests.11
| Test | Approximate normal range12 | Description | |
|---|---|---|---|
 | Platelet count13,14 | 150–350 x 109 cells/L | There are four main methods of counting the number of platelets in the blood:
|
| Activated partial thromboplastin time (aPTT)14 | 25–35 seconds (ratio to normal: 0.8–1.2) | A measure of the time necessary to generate fibrin via the intrinsic pathway of the coagulation cascade. Assesses the levels of factors XII, IX, VIIII, X, V, prothrombin, and fibrinogen. |
| Prothrombin time (PT)15 | 11–13 seconds (ratio to normal: 0.8–1.2) | A measure of the time necessary to generate fibrin via the extrinsic pathway of the coagulation cascade (i.e., levels of factors VII, V, X, prothrombin, and fibrinogen). The international normalised ratio (INR) can be derived from the PT for patients receiving vitamin K antagonists as a way to harmonise results across different thromboplastins16. |
| Thrombin time14 | 18.5–24 seconds | A measure of the rate of fibrinogen conversion to fibrin in the presence of thrombin. Identifies changes in the levels of fibrinogen and if there are any inhibitors to fibrinogen activation. |
| Fibrinogen assay17-19 | 1.5–3.5 g/L | The level of fibrinogen is assessed based on clotting time of diluted plasma exposed to high concentrations of thrombin, which is then compared against a standard calibration curve. There are two main methods for assessing fibrinogen levels:
|
| Activated clotting time20 | Variable | A commonly used assay for monitoring anticoagulation during extracorporeal life support. The assay uses whole blood, so also includes measurement of platelets and phospholipids. |
Fibrinolysis is difficult to measure directly using conventional laboratory tests.7 Standardised methods to detect abnormalities in fibrinolysis are not as simple or widely available as those used to measure blood clotting.21–23
Assessment of euglobulin clot lysis time can be used as a measure of fibrinolysis. Alternatively, antigen tests for plasma proteins involved in fibrinolysis, such as tissue plasminogen activator (tPA), plasminogen activator inhibitor 1 (PAI-1) and thrombin-activatable fibrinolysis inhibitor (TAFI) can be assessed.21–23 Fibrin degradation products, such as D-dimers, are another parameter that can be measured.
D-dimers are late-stage fragments released when fibrin, cross-linked by factor XIII, is degraded by plasmon.9 Circulating D-dimer levels are therefore a measure of fibrin breakdown and ongoing fibrinolysis.9,12 In a clinical setting, D-dimer tests are restricted to the exclusion of thrombosis, due to their low specificity.21,22
The process of obtaining laboratory tests can be time consuming and often takes too long to be able to provide real-time guidance in a clinical setting.11,23 Haemostasis is a complex process in vivo, and consequently individual laboratory tests only give a narrow view of the clinical picture. They are often performed on plasma samples without platelets and other blood cells, and they do not always differentiate between the various pathological mechanisms that may be contributing to a specific clinical situation.11 Some tests are also susceptible to interference from particular serum constituents or antibodies,8,21 and some comorbidities, inflammatory mediators or commonly administered treatments can also adversely affect the sensitivity of certain assays.7,11
