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→Prothrombin Time
===Prothrombin Time===
===Prothrombin Time===
Prothrombin time (PT) gives an assessment of the extrinsic and common pathways by measuring the time necessary to generate fibrin after activation of factor VII<sup>3</sup>. It is performed by an automated analyser<sup>2</sup> using citrated plasma<sup>1, 3</sup>. Blood should therefore be collected into a sodium citrate tube if prothrombin time is to be performed. In basic terms, the test procedure involves adding thromoplastin to the patient's plasma, warming, adding calcium and recording the time taken to clot<sup>1</sup>.
Prothrombin time (PT) gives an assessment of the extrinsic and common pathways by measuring the time necessary to generate fibrin after activation of factor VII<sup>3</sup>. It is performed manually or by an automated analyser<sup>2</sup> using citrated plasma<sup>1, 3</sup>. Blood should therefore be collected into a sodium citrate tube if prothrombin time is to be performed. As a quality control measure it is normal to undertake the test in a sample from an unaffected patient to compare the time taken to clot between the two samples - this will account for variables such as variations in the technique of performing the manual test. In basic terms, the test procedure involves adding rabbit brain thromoplastin to the patient's plasma at 37 Degrees and recording the time taken for the sample to clot<sup>1</sup>.
A prolonged PT may reflect a factor deficiency or the presence of a circulating inhibitor of coagulation. Repeating the test using a mix of test plasma and "normal" plasma can help differentiate these possibilities: PT returns to normal limits when normal plasma is added to factor-deficient plasma, but no change is seen when this is added to plasma containing inibitors<sup>3</sup>. PT is more sensitive than APTT for factor deficiencies.
A prolonged PT may reflect a factor deficiency or the presence of a circulating inhibitor of coagulation. Repeating the test using a mix of test plasma and "normal" plasma can help differentiate these possibilities: PT returns to normal limits when normal plasma is added to factor-deficient plasma, but no change is seen when this is added to plasma containing inibitors<sup>3</sup>. PT is more sensitive than APTT for factor deficiencies.