D-dimer is a specific indicator of thrombosis and is very sensitive, there are different clinical tests, some hospitals are 500μg/L, 500μg/L is the upper limit of normal value, more than 500μg/L should be considered the possibility of thrombosis. dimer may be elevated. Therefore, it is important to combine elevated D-dimer indicators with clinical conditions and ultrasound findings to make a comprehensive judgment. In addition, the dimer can be used to determine whether the thrombus is old or fresh. Patients with symptoms of swelling and pain and elevated dimer, combined with ultrasonography, are considered to have fresh thrombosis. In fresh thrombosis, the dimer will be very much elevated, often more than ten or even tens of times, and through treatment, the dimer will gradually return to normal. If the dimer is normal or gradually decreases, swelling and pain disappear, it means that the thrombus is more stable and can be moved to the ground normally.