Oral anticoagulant therapy has become more commonly prescribed for people of 65 years and over following the widening of its clinical indications to include thromboembolic prophylaxis in atrial fibrillation. It is estimated that approximately 470 000 people in the UK, almost 1% of the population, are currently receiving oral anticoagulant therapy and this is growing. Due to its narrow therapeutic index, and the intra-individual variation in dose requirement, there is a need to monitor the level of anticoagulation in each patient. In consequence, health systems have had to invest resources into monitoring services to cope with this development.