Background: Atrial fibrillation (AF) is the most common heart rhythm disorder worldwide. Globally, there are 37.5 million people with AF. AF increases the risk of death, heart failure, and stroke, which severely affect patients and also lead to high healthcare costs. Around 25% of all strokes are caused by AF, and patients with stroke due to AF tend to have a higher risk of death and more disability compared to stroke patients without AF. Stroke prevention is, therefore, an important part of AF treatment, in which blood thinning medication has an important role. However, blood thinners increase the risk of bleeding. Therefore, it is important to divide patients with AF into different risk groups, known as risk assessment, to figure out who will benefit the most from blood thinners. To be able to divide patients into different risk groups, various stroke risk assessment tools have been developed, such as the CHA2DS2-VASc score and the ABC-stroke score. The most commonly used tool is the CHA2DS2-VASc score, including only clinical risk factors, such as high blood pressure, diabetes, etc. The ABC-stroke score, which includes blood markers of heart function, has been proven to outperform the CHA2DS2-VASc score in terms of predicting stroke in AF patients. However, the CHA2DS2-VASc score remains the primary stroke risk assessment tool for AF patients in current guidelines. After looking at the different risk factors, patients are divided into three groups: low, intermediate, and high risk. High-risk patients must take blood-thinning medication for life, while low-risk patients do not need it. In the medium-risk group, it remains uncertain whether blood thinners should be given or not. Despite the broad use of the CHA2DS2-VASc score, the score itself has limitations. The score does not include important factors, such as the duration of AF, the size and function of the upper heart chambers, as well as the stiffness of the heart, and markers of blood clotting, which are proven markers of a state that inceases the risk of blood clots. Furthermore, the CHA2DS2-VASc score does not consider whether heart failure, high blood pressure, and diabetes are well-controlled or not, which could lead to overuse of blood thinners. Therefore, the current risk assessment tools for patients with AF are incomplete, and improvements are needed. Overall hypothesis: Overall hypothesis is that the different components of the CHA2DS2-VASc score and ABC-stroke score affect blood clotting markers differently, depending on whether conditions like heart failure, high blood pressure, and diabetes (modifiable risk factors) are well-controlled or not. Investigators also expect to see differences in blood clotting markers across different stroke risk groups (low, intermediate, and high risk, based on the CHA2DS2-VASc score and ABC-stroke score) in AF patients who have not yet started blood thinning medication. Furthermore, investigators believe that the duration of AF, the size/function of the upper heart chambers, as well as the stiffness of the heart, can reflect an increased risk of blood clots in AF patients. Overall goal of the study: The overall goal of the study is to help improve the current tools used to assess the risk of stroke in patients with newly diagnosed AF. This will be done by adding more factors to the current risk assessment tools that reflect an increased risk of stroke, such as the burden of AF, the size/function of the heart's upper chambers, as well as the stiffness of the heart, and using biomarkers that show the blood's ability to clot as a substitute measure for stroke risk.

Methods: The study is a cross-sectional, single-center observational study and will take place at Esbjerg Hospital - University Hospital of Southern Denmark, involving collaboration between the Unit for Thrombosis Research, Department of Clinical Diagnostics and the Department of Cardiology. The study population will consist of 150 participants with newly diagnosed AF. The participants must not be taking a specific type of blood thinner, called anticoagulant therapy (OAC-therapy), before being included in the study. The participants will be recruited with the help of the general practitioners (GPs). The general practitioners will be thoroughly informed about the study and the importance of waiting to start OAC-therapy until the participants have been seen at the cardiology outpatient clinic. The participants will be scheduled for a blood test, an ultrasound of the heart (echocardiography), and a 7-day heart rhythm monitoring within 4 days after their first meeting with the GP.