![]() ![]() ![]() von Willebrands disease (as vWF pairs up with factor VIII).Haemophilia C (XI – autosomal recessive).Haemophilia B (IX – X-linked recessive).Haemophilia A (VIII – X-linked recessive).The main conditions that could result in an abnormal APTT include: Like with PT, APTT time will be affected by overall clotting factor synthesis or consumption (it can also be affected by DIC, liver failure, vitamin K deficiency and warfarin levels).ĪPTT, however, can indicate issues with factors VIII (and vWF), IX, and XI specifically. The activated partial thromboplastin time ( APTT) is a measure of the time taken for blood to clot via the intrinsic pathway (a good way to remember, following PT, is that you ‘ Play Table Tennis INSIDE’ therefore PTT is INTRINSIC). This test can be affected by liver disease, disseminated intravascular coagulation (DIC), vitamin K deficiency and warfarin levels. Since the only factor in the pathway is VII (which is rarely deficient in isolation), these tests are a measure of overall clotting factor synthesis or consumption. International normalised ratio (INR) is a standardised version of this test, commonly used with patients on anticoagulants. The prothrombin time ( PT) is a measure of the time taken for blood to clot via the extrinsic pathway (a good way to remember is that you ‘ Play Tennis OUTSIDE’ therefore PT is EXTRINSIC). There several components to the coagulation screen, some of the core parts and what they mean are covered below. ![]() This plasmin then breaks down the fibrin mesh. Blood coagulation pathway 1 How are clots later dissolved?įibrin clots can then be dissolved through a separate pathway whereby plasminogen becomes its activated form, plasmin, via tPA. The diagram below should help put all of this into context. The combined pathway then proceeds the intrinsic/extrinsic pathways and goes as follows:įactor X (+ Factor V) → Factor II → Factor I (+ Factor XIII → Fibrin mesh) The extrinsic pathway is activated by the release of tissue factors and goes as follows:įactor VII → Common pathway Combined pathway The intrinsic pathway is activated by contact with a damaged surface and goes as follows:įactor XII → Factor XI → Factor IX (+ Factor VIII + vWF) → Common pathway Extrinsic pathway Key features of each pathway are shown below. These factors form 3 groups: the intrinsic pathway, the extrinsic pathway, and the combined pathway. The clotting cascade is comprised of many different factors, ranging from I through to XIII. Now, let’s look at how the clotting cascade works. These newly expressed receptors then finally bind to fibrinogen, which leads to further platelet aggregation. After binding, the ADP (P2Y12) receptor on the platelet is activated leads to increased expression of GpIIb/IIIa.Ĥ. vWF then binds to GpIb receptors on surrounding platelets.ģ. von Willebrand’s Factor (vWF) adheres to the vascular injury.Ģ. In response to vascular injury, the following steps lead to a platelet plug:ġ. First, we shall look at how platelets work. The coagulation screen assesses the function of the clotting cascade and the body’s ability to create a platelet plug. You might also be interested in our premium collection of 1,000+ ready-made OSCE Stations, including a range of test result interpretation stations ✨ How does coagulation work?
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