Laboratory evaluation of haemostatic function Global tests of haemostatic function

Defective haemostasis with abnormal bleeding may result from thrombocytopenia (quantitative platelet defect), platelet function disorder (qualitative platelet defect), or defective blood coagulation. Patients with a variety of vascular disorders may also suffer from a bleeding disorder. In association with the clinical picture, including family history, the haemostatsic function may be evaluable using a number of initially simple tests to assess the platelet, vessel wall, and coagulation components of haemostasis.19

Blood count and blood film examination

As thrombocytopenia is a common cause of abnormal bleeding, patients with suspected bleeding disorders should initially have a blood count, including platelet count and blood film examination to exclude platelet aggregation as a cause of false throm-bocytopenia and confirm the presence of thrombocytopenia. The cause of thrombocytopenia may be obvious, e.g., acute leukaemia or DIC.20

Table 2.

Screening Tests for Coagulation Disorders

Screening Test

Abnormality Indicated by

Most Common Cause

Prolongation

of Abnormality

Prothrombin time (PT)

Deficiency or inhibition of

Liver disease

one or more of the

Warfarin therapy

following coagulation

or other vitamin K

factors: VII, X, V, II,

antagonists

fibrinogen

Activated partial

Deficiency or inhibition of

Haemophilia A

thromboplastin

one or more of the

Haemophilia B

time (APTT)

following coagulation

von Willebrand's

factors: XII, XI, IX, VIII, X,

disease

V, II, fibrinogen

Lupus

anti-coagulants

Acquired

haemophilia and

heparin therapy

Thrombin time (TT)

Reduction or abnormality of

Disseminated

fibrinogen or inhibition of

intravascular

thrombin by heparin or

coagulation (DIC)

FDPs

Heparin therapy

Screening tests of blood coagulation

Screening tests of blood coagulation

These may provide an assessment of the different components of the extrinsic, intrinsic, and/or common pathway of blood coagulation (Table 2). Generally, a significant prolongation of the respective clotting tests beyond those of normal 'control' plasmas in the test system indicates a defect.

The prothrombin time (PT ) measures the extrinsic system (factor VII) and factors common to both systems (factors X, V, prothrombin, and fibrinogen). It may be expressed as the international normalised ratio (INR). The activated partial thromboplastin time (APTT) measures the intrinsic system (factors VIII, IX, XI, and XII) in addition to factors common to both systems (factors X, V, prothrombin, and fibrino-gen). The thrombin time (TT) is sensitive to abnormalities of fibrinogen (quantitative or qualitative) or to inhibition of thrombin.19,20

Mixing equal volumes of the test plasma with normal plasma will correct a prolonged PT or APTT due to factor deficiency. The presence of an inhibitor of coagulation is suspected when there is incomplete or no correction.19,20

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