LDL 3.9mmol/L is a borderline level and is not serious, but it needs to be controlled, and if it continues to rise it can cause coronary heart disease and other more serious conditions. LDL is one of the forms of serum lipids. Normal adult LDL level should be less than 3.4mmol/L, between 3.4mmol/L and 4.1mmol/L is borderline level, and more than 4.1mmol/L is elevated. 3.9mmol/L is a borderline level, which is not serious, but needs to be controlled in time to avoid further elevation. Common control measures include a light diet to avoid excessive lipid intake, moderate exercise, and weight loss for those who are overweight. If the borderline level continues to rise above 4.1 mmol/L, it will lead to an increased risk of coronary heart disease, atherosclerosis and other diseases, which is more serious and requires timely treatment. If there are high-risk factors such as coronary heart disease, blood lipids need to be strictly controlled within the range below the borderline level; the presence of abnormally elevated LDL is recommended to go to the hospital in a timely manner in order to avoid delaying the condition.