Before starting oral anticoagulant treatment it is advisable to perform the first line coagulation screen: a prothrombin time (PT), an activated partial thromboplastin time (APTT) and a platelet count. Any abnormality of these tests must be investigated because a contraindication to the use of oral anticoagulants may be revealed and an abnormality will confound their use for controlling anticoagulant effect. History and clinical examination should be assessed to ensure that no local or general haemorrhagic diathesis exists.

Methods used for the laboratory control of oral anticoagulant treatment
The one-stage PT of Quick is the most commonly used test. Originally, lack of standardisation of the thromboplastin preparations and methods of expressing the PT results led to great discrepancies in the reported results and hence also in anticoagulant dosage. The use of the International Sensitivity Index (ISI), to assess the sensitivity of any given thromboplastin, and the International Normalised Ratio (INR), to report the results, has minimised these difficulties and greatly improved uniformity of anticoagulation throughout the world.