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Coagulation tests: Difference between revisions
→Prothrombin time and international normalised ratio (INR)
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The problem with prothrombin time is that the result varies significantly from lab to lab, depending on equipment and substrates used. As such, the prothrombin time is rarely evaluated alone, but the INR is used instead. | The problem with prothrombin time is that the result varies significantly from lab to lab, depending on equipment and substrates used. As such, the prothrombin time is rarely evaluated alone, but the INR is used instead. | ||
<section begin="pharmacology" /> | |||
The '''international normalised ratio''' (INR) is a standardised form of prothrombin time which is normalised so that the result is similar between different laboratory methods and equipments. The prothrombin time is first measured and then normalised by a specific equation and factor. INR is generally used instead of prothrombin time for the same indications.<section end="physiology" /> | The '''international normalised ratio''' (INR) is a standardised form of prothrombin time which is normalised so that the result is similar between different laboratory methods and equipments. The prothrombin time is first measured and then normalised by a specific equation and factor. INR is generally used instead of prothrombin time for the same indications.<section end="physiology" /> | ||
<section end="pharmacology" /> | |||
Indications: | Indications: | ||
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=== Warfarin === | === Warfarin === | ||
Warfarin is a vitamin K antagonist used as an anticoagulant. It was previously widely used, but the direct oral anticoagulants have replaced warfarin for most indications. | Warfarin is a vitamin K antagonist used as an anticoagulant. It was previously widely used, but the direct oral anticoagulants have replaced warfarin for most indications. | ||
<section begin="pharmacology" /> | |||
When warfarin is used for atrial fibrillation, the warfarin dose should be adjusted so that the INR is between 2.0 and 3.0. When warfarin is used for mechanical heart valves, the INR should rather be between 2.5 and 3.5. | When warfarin is used for atrial fibrillation, the warfarin dose should be adjusted so that the INR is between 2.0 and 3.0. When warfarin is used for mechanical heart valves, the INR should rather be between 2.5 and 3.5.<section end="pharmacology" /> | ||
=== Liver failure === | === Liver failure === | ||
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Several coagulation factors can be congenitally or acquired deficient. The PT and INR are prolonged in case of deficiency of fibrinogen (factor I) and factors II, V, VII, or X. | Several coagulation factors can be congenitally or acquired deficient. The PT and INR are prolonged in case of deficiency of fibrinogen (factor I) and factors II, V, VII, or X. | ||
<section begin="physiology" /> | <section begin="physiology" /> | ||
== Activated partial thromboplastin time (aPTT) == | == Activated partial thromboplastin time (aPTT) == | ||
The '''activated partial thromboplastin time''' (aPTT) evaluates the intrinsic and common pathways of coagulation. It's measured by taking the time it takes for plasma to clot when exposed to a reagent which contains phospholipids, silica, and a thromboplastic material without tissue factor. This material activates contact factor, initiating the intrinsic pathway. | The '''activated partial thromboplastin time''' (aPTT) evaluates the intrinsic and common pathways of coagulation. It's measured by taking the time it takes for plasma to clot when exposed to a reagent which contains phospholipids, silica, and a thromboplastic material without tissue factor. This material activates contact factor, initiating the intrinsic pathway. | ||