Coagulation monitoring Basic coagulation screen

The basic screen consists of a platelet count, prothrombin time, activated partial thromboplastin time and thrombin time. Close attention to blood sampling technique is very important for correct interpretation of coagulation tests. Drawing blood from indwelling catheters should, ideally, be avoided since samples may be diluted or contaminated with heparin. The correct volume of blood must be placed in the sample tube to avoid dilution errors. Laboratory coagulation tests are usually performed on citrated plasma samples taken into glass tubes.

Specific coagulation tests Activated clotting time (ACT)

Sample tube contains celite, a diatomaceous earth, which activates the contact system; thus the ACT predominantly tests the intrinsic pathway. The ACT is prolonged by heparin therapy, thrombocytopenia, hypothermia, haemodilution, fibrinolysis and high dose aprotinin. Normal is 100-140s.

Thrombin time (TT)

Sample tube contains lyophilised thrombin and calcium. Thrombin bypasses the intrinsic and extrinsic pathways such that the coagulation time tests the common pathway with conversion of fibrinogen to fibrin. The TT is prolonged by fibrinogen depletion, e.g. fibrinolysis or thrombolysis, and heparin via antithrombin III dependent interaction with thrombin. A high dose TT is more sensitive to heparin anticoagulation than fibrinogen levels. Normal range is 12-16s.

Prothrombin time (PT)

Sample tube contains tissue factor and calcium. Tissue factor activates the extrinsic pathway. The PT is prolonged with coumarin anticoagulants, liver disease and vitamin K deficiency. Normal range is 12-16s. The International Normalised Ratio (INR) relates PT to control and is normally 1.

Activated partial thromboplastin time (APTT)

Sample tube contains kaolin and cephalin as a platelet substitute to activate the intrinsic pathway. The APTT is prolonged by heparin therapy, DIC, severe fibrinolysis, von Willebrand factor, factor VIII, factor X1 or factor XIII deficiencies. Normal range is 30-40s.

D-dimers and fibrin degradation products (FDPs)

Fibrin fragments are released by plasmin lysis. FDPs can be assayed by an immunological method; they are often measured in the critically ill to confirm disseminated intravascular coagulation. A level of 20-40pg/ml is common post-operatively, in sepsis, trauma, renal failure and DVT. Raised levels do not distinguish fibrinogenolysis and fibrinolysis. Assay of the d-dimer fragment is more specific for fibrinolysis, e.g. in DIC, since it is only released after fibrin is formed.

Coagulation factor assays

Assays are available for all coagulation factors and may be used for diagnosis of specific defects. As heparins inhibit factor Xa activity, the factor Xa assay is therefore the most specific method of controlling low molecular weight heparin therapy. Since this assay is not dependent on contact system activation, it also avoids the effects of aprotinin when monitoring heparin therapy.

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