This prospective observational study is to assess the predictive factors of poor control of international normalized ratio (INR) to determine the treatment strategies received by patients with a poor control of INR in real-life clinical practice and to explore the effectiveness and safety of these strategies in patients with non-valvular atrial fibrillation who start vitamin K antagonist (VKA) treatment

Inclusion Criteria:

• Patients aged ≥ 18 years.
• Patients with a diagnosis of nonvalvular atrial fibrillation according to the criteria of the Spanish Agency of Medicines and Medical Devices (AEMPS) (2013) who are treated in cardiology practices.
• Patients about to start treatment with a vitamin K antagonist or who are in the first two months of treatment.
• Patients for whom access to at least 80% of INR tests performed during treatment with vitamin K antagonists is expected to be available.
• Patients with the mental and physical capacity to complete the study questionnaires and give their informed consent to participate in the study.

Exclusion Criteria:

• Patients with mitral stenosis or another significant valve disease for which specific treatment is scheduled or has already been performed (prosthesis or valvuloplasty).
• Patients hospitalised at the time of inclusion in the study.
• Patients with a life expectancy of less than 13 months.
• Patients who are participating in a clinical trial.
• Patients receiving double antiplatelet therapy.

Adult patients diagnosed with non-valvular atrial fibrillation who are going to start vitamine K antagonist treatment or have started over the previous two months and that attend the Cardiology Units at Spanish hospitals (as outpatients).