Difference between prothrombin time, INR and Activated partial thromboplastin time?

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MedStudentt
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So I have searched this and although the definitions are written slightly different, they all seem to mean round about the same thing to me.

Could anyone explain the clinical difference of these terms?
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Democracy
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(Original post by MedStudentt)
So I have searched this and although the definitions are written slightly different, they all seem to mean round about the same thing to me.

Could anyone explain the clinical difference of these terms?
Clinically this is what's important:

APTT = measures the intrinsic pathway of clotting, typically expressed in seconds. Clinically you use it to measure the activity of unfractionated (IV) heparin and to adjust the rate of infusion accordingly (NB: it does not measure the activity of LMWHs like enoxaparin).

PT = measures the extrinsic pathway of clotting (factors II, VII, IX, X), typically expressed in seconds. This will be prolonged in a patient on warfarin, since warfarin inhibits vitamin K epoxide reductase which is normally involved in the process which makes those factors functional and able to coagulate. This is why the PT is used to calculate the INR and why vitamin K can be used to (slowly) reverse an excessively high PT/INR. PT is also a test of true liver function since the clotting factors are synthesised by the liver - which is why it will be deranged in chronic liver disease and hepatic failure.

INR = a ratio which again looks at the extrinsic pathway of coagulation which compares your patient's PT to a control PT (i.e. it doesn't have a unit). An INR of 2.0 means your patient's blood has taken twice as long to coagulate compared to the control. For anticoagulation purposes you use the INR to monitor how adequately someone is being warfarinised - you can't use it to get an idea of anticoagulation on NOACs or heparins.

APTT and PT also come up in questions on coagulopathies/haemophilias/thrombophilias, so it might be worth looking those up once you've grasped the difference between the tests.
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nexttime
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You can also measure bleeding time, and it's NOT just the sum of other clotting abnormalities, just to confuse you further ; )
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Asklepios
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(Original post by Democracy)

PT = measures the extrinsic pathway of clotting (factors II, VII, IX, X),
Factors II & X are common pathway, and factor IX is intrinsic pathway. Only factor VII is extrinsic!

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