There has been a substantial increase in the use of oral anticoagulants, notably in the ageing population. This is primarily driven by the use of warfarin in people with atrial fibrillation (AF) in order to reduce the risks of thromboembolic events such as stroke.1 In elderly patients with AF, warfarin use has been shown to reduce the relative risk of stroke by >50% compared with aspirin alone.2 Warfarin is also used to treat patients with deep vein thrombosis, mechanical heart valve replacement, cardioversion, cardiomyopathy and antiphospholipid syndrome.1

The clinical benefits of warfarin must also be balanced with potential risks. Over-dosage can cause major haemorrhage while under-anticoagulation may lead to thrombosis. As a result of these risks, warfarin therapy is monitored regularly using the international normalised ratio (INR) to prevent under-anticoagulation or over-anticoagulation. For example, a target INR of 2.0–3.0 is recommended for the prevention of thromboembolic events in AF and an INR of 3.0–4.0 for mitral valve replacement.1

Maintaining individuals within the narrow target ranges for INR can, however, prove challenging in routine clinical practice. The percentage of time in target range (TTR) for INR is an important marker of control, with a clear association between a low TTR and adverse events. In recent guidance on the management of anticoagulation in AF (CG180), the National Institute for Health and Care Excellence (UK) has suggested reassessing patients with poor INR control when TTR is <65%.1

Several factors can affect INR variability, most notably dietary-related and drug–drug interactions. Medications (such as macrolide and …