Ankle-brachial pressure index (ABPI): what the test shows

The NHS describes the ankle brachial pressure index (ABPI) test as widely used to diagnose peripheral arterial disease and to assess how well you are responding to treatment. PAD is a common condition in which a build-up of fatty deposits in the arteries restricts blood supply to the leg muscles.

The test works on a simple principle. Blood pressure measured at the ankle should be about the same as at the arm. If the leg arteries are narrowed, less pressure reaches the ankle, so the ankle pressure falls compared with the arm.

The NHS says to see a GP if you have recurring leg pain when exercising, because many people mistakenly think it is just part of growing older. PAD is usually diagnosed through a physical examination and by comparing the blood pressure in your arm and your ankle.

NICE guideline CG147 says people should be assessed for PAD if they have symptoms suggestive of it, or they have diabetes, non-healing wounds on the legs or feet, or unexplained leg pain, or if interventions to the leg or foot or compression hosiery are being considered. Assessment includes asking about symptoms of intermittent claudication and critical limb ischaemia, examining the legs and feet, feeling the pulses and measuring the ABPI.

You lie on your back. The NHS says the blood pressure in your upper arms and ankles is measured using a cuff and a Doppler probe, which uses sound waves to determine the blood flow in your arteries, and the ankle result is divided by the arm result.

NICE describes the method in more detail. The person should be resting and lying down if possible. Systolic blood pressure is recorded with an appropriately sized cuff in both arms and in the arteries at the ankle and foot, and measurements should be taken manually using a Doppler probe in preference to an automated system. The nature of the Doppler signals in the foot arteries is documented. The index for each leg is calculated by dividing the highest ankle pressure by the highest arm pressure.

Because it relies on cuffs and a Doppler probe rather than needles, it is generally a simple, non-invasive check.

The NHS says that if your circulation is healthy the pressure in both parts of your body should be exactly or almost the same, which would make the result 1. If you have PAD, the pressure in the ankle will be lower because of a reduction in blood supply, making the ABPI less than 1.

The NHS and NICE pages read for this guide do not list a cut-off for each grade of severity, so this guide does not give one. Your result is interpreted by the clinician who requests it, alongside your symptoms and the examination of your legs and feet. The NHS adds that if you have leg pain but your ABPI score is normal, or you do not fit the expected profile, for example you are younger than 60 and have never smoked, further testing may be used.

NICE explains that evidence showed ABPI below an agreed cut-off increases the probability of diagnosing PAD, but that people with diabetes and PAD may have a normal or raised index because of hardening of the arteries. It therefore recommends not excluding PAD in people with diabetes on a normal or raised ABPI alone, and recommends against using pulse oximetry for diagnosing PAD in people with diabetes.

In practice this means a normal ABPI in someone with diabetes who has foot ulcers or other signs does not end the investigation. NICE mentions Doppler waveform analysis and the toe brachial index as other tests, noting there was not enough evidence to make recommendations on their use in this group.

The NHS says in most cases a GP can confirm PAD from the examination, symptoms and ABPI. Further testing is usually only needed if there is uncertainty or if the restriction of blood supply is severe enough that surgery may be required. Hospital tests may include an ultrasound scan to identify where arteries are blocked or narrowed, and an angiogram using contrast on a CT or MRI scan.

The NHS describes PAD as largely treated by lifestyle changes and medicines, with exercise and not smoking the main lifestyle changes, and says the underlying causes such as high blood pressure, high cholesterol and diabetes should also be treated. It advises contacting a GP immediately, or 111 if that is not possible, if you think you are developing symptoms of critical limb ischaemia, such as severe burning pain in the legs and feet at rest or wounds that do not heal.