Answer: A
PT measures the effectiveness of the extrinsic coagulation. It is performed by measuring the time it takes to form a clot when calcium and a tissue factor are added to the patient’s plasma. A normal PT indicates normal levels of factor VII and those factors common to both the intrinsic and extrinsic pathways (V, X, Prothrombin(II) and Fibrinogen (I)). Prolongation of less than one and a half times the control value is usually associated with mild bleeding disorders, while further prolongation indicates a more severe bleeding disorder. The PT is used to monitor oral anticoagulant therapy.
International Normalized Ratio (INR): For patients on chronic oral anticoagulants, to report the PT ratio. Using an international sensitivity index (ISI) which is determined for each batch of reagent and is specific to the lab’s particular equipment, the calculation of INR= (patient PT/normal PT) ISI . For most conditions that require ongoing anticoagulation therapy, AHA has recommended INR levels between 2.0 and 3.0.
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