Interpretation of results of screening tests used in the diagnosis of coagulation disorders
Screening tests | Abnormalities indicated by prolongation of screening test result | Examples of causes of prolongation of screening test |
thrombin time - this test is sensitive to a deficiency of fibrinogen or inhibition of thrombin. | abnormalities in thrombin time indicate an abnormality or deficiency of fibrinogen, OR, inhibition of thrombin by heparin or fibrin degradation products (FDPs) | disseminated intravascular coagulation heparin therapy |
prothrombin time - measures the extrinsic system (also known as the tissue factor pathway) (factor VII) as well as factors that are common to both systems (factor X, V, prothrombin and fibrinogen). The normal prothrombin time for clotting is 10-14 seconds. It may be expressed as the international normalized ratio (INR). | abnormalities in prothrombin time indicate an inhibition or deficiency of one or more of the following coagulation factors: factor II (prothrombin), V, VII, X, fibrinogen The speed of the extrinsic pathway is greatly affected by levels of factor VII in the body. Factor VII has a short half-life and it requires vitamin K for synthesis. | Deficiencies in vitamin K, which can be caused by warfarin, malabsorption or lack of intestinal colonization by bacteria (such as in newborns) Reduced factor VII synthesis (due to liver disease) Increased consumption (in disseminated intravascular coagulation) |
activated partial thromboplastin time (APTT) - this test measures the intrinsic system (also known as the contact activation pathway) factors VIII, IX, XI and XII as well as factors common to both systems (factors X, V, prothrombin and fibrinogen). Note that the term partial is used for this screening test due to the absence of tissue factor from the reaction mixture. | abnormalities in APTT indicate an inhibition or deficiency of one or more of the following factors: XII, IX, VIII, XI, X, V, II, fibrinogen | Haemophilia Other causes include:
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