A Doppler test uses ultrasonic waves to assess the movement of blood through the blood vessels. It is used as a preventative measure for conditions such as stroke. The Doppler test usually used on the major veins and arteries of the legs, arms and neck. It is also used to locate blood clots that, if they break loose, can be fatal. During a Doppler test, a device is held over the skin. This device sends and picks up sound waves. The movement of the blood causes changes in pitch.