Clotting screen
From WardWiki - Foundation Doctor Helper
A clotting screen provides a baseline assay of coagulation ability of the blood as measured by PT, APPT and Fibrinogen. The topic of coagulation is vast and this article focuses on sample collection and ward based interpretation of results. It is a basic blood test.
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Sample collection
Blood will start to coagulate is is drawn from the body and the citrated sample bottle must be adequately filled and be filled quickly to prevent in vitro activation of coagulation. Clotting screens are indicated where vitamin k metabolism is altered by coumarin anticoagulation or biliary obstruction, or when sepsis or large blood loss is present or expected and requires additional clotting factors. They are an essential component of coumarin anticoagulation monitoring and failure to sample a patient may proclude its use.
Prothrombin time
PT measures the extrinsic pathway of coagulation, namely the clotting factors I, II, V, VII, and X. It is a measure of liver function and is affected by coumarin anticoagulation. An internationalised ratio of PT is the International Normalised Ratio, INR. A high INR can be dangerous and it is a common problem on wards. PT is raised by heparin but to a lesser degree. Range 9 - 14 seconds.
Activated partial thromboplastin time
APTT measures the intrinsic pathway of coagulation, namely the clotting factors high-molecular-weight kininogen, XII, XI, VIIIa and X. It is used clinically as a monitor of the effects of heparin, which also raises PT to a lesser degree. An APPT ratio monitors heparin in a conceptually similar manner to INR. Range 23-34 seconds (varies between labs)
Fibrinogen
Fibrinogen is a clotting factor synthesized in the liver. It is nominally clotting factor I but seldom referred to as this in clinical practice. It is cleaved to form fibrin, a key component of thrombus. It is raised in sepsis and their pathologies making it an acute phase reactant. A high fibrinogen is entirely appropriate in sepsis and cancer, but a low and falling fibrinogen is an ominous hallmark of DIC and warrants haematology advice.
Time taken for result
Usually within 2 hours; 45 minutes if urgent
