A plain description of a test is not an invitation to run it yourself. The ankle-brachial index sounds simple, which is exactly why a reader in Tinley Park, Illinois should slow down before treating it as a home project.
What the test compares
www.nhlbi.nih.gov puts it this way: "The test compares blood pressure in your ankle with the blood pressure in your arm." It is a comparison. A single number from one limb is not the comparison that sentence describes.
Tests with similar-sounding names
Leg circulation has other tests that are easy to confuse with this one. The MedlinePlus Medical Encyclopedia has an entry on the Doppler ultrasound exam of an arm or leg. Reading the two descriptions side by side shows that they are different tests with different names.
Where self-reading stops
You can learn what a test is called and what it compares. You cannot judge from a description what your own numbers mean. Interpreting them is a clinician's job.
Other vessels, other tests
The neck has its own test. MedlinePlus describes it in an entry on carotid duplex. It is mentioned here so that you do not mix it up with the ankle and arm comparison.
The conservative choice
If you are tempted to take or compare pressures on your own, ask a clinician first whether that is useful for you. Do not act on a number you cannot interpret.
Two neighboring questions to keep separate
Leg swelling is a separate subject from this test. MedlinePlus has an encyclopedia entry on deep vein thrombosis for background.
The NHLBI, NIH also has a page on diagnosing an aortic aneurysm, which is a different condition and a separate question.