Non Valvular Atrial Fibrillation (nv AF)

The goal of this observational study is to learn about the variability of apixaban blood levels and identify the clinical factors that influence them in Asian patients with atrial fibrillation. The main questions it aims to answer are: 1. What are the specific pharmacokinetic parameters and their variabilities for apixaban in this patient population? 2. Which clinical factors, such as kidney function or body weight, are the primary predictors of how apixaban is cleared and distributed in the body? Participants will be asked to give consent to allow researchers to use their existing electronic outpatient medical records and previously collected blood concentration data for analysis.