The ankle-brachial index, or ABI, is a simple test that relates the blood pressure of the top and lower limbs. Healthcare professionals measure ABI by dividing blood pressure in the artery of the ankle with blood pressure in the artery of the arm. The result of this is the ABI. If this ratio is less than 0.9, it may mean that an individual has peripheral artery disease (PAD) in his or her blood vessels. Plaque constructs up in the arteries in PAD. It often impacts the vessels that carry blood to the legs. Decreased blood flow can cause pain and numbness. Low ABI may mean that your legs and legs don't get as much blood as they need. The ABI test will not help illustrate which blood vessels have been narrowed or obstructed, though.