Ankle–brachial pressure index
The ankle-brachial pressure index or ankle-brachial index is the ratio of the blood pressure at the ankle to the blood pressure in the upper arm. Compared to the arm, lower blood pressure in the leg suggests blocked arteries due to peripheral artery disease. The ABPI is calculated by dividing the systolic blood pressure at the ankle by the systolic blood pressure in the arm.
Method
The patient must be placed supine, without the head or any extremities dangling over the edge of the table. Measurement of ankle blood pressures in a seated position will grossly overestimate the ABI.A Doppler ultrasound blood flow detector, commonly called Doppler wand or Doppler probe, and a sphygmomanometer are usually needed. The blood pressure cuff is inflated proximal to the artery in question. Measured by the Doppler wand, the inflation continues until the pulse in the artery ceases. The blood pressure cuff is then slowly deflated. When the artery's pulse is re-detected through the Doppler probe the pressure in the cuff at that moment indicates the systolic pressure of that artery.
The higher systolic reading of the left and right arm brachial artery is generally used in the assessment. The pressures in each foot's posterior tibial artery and dorsalis pedis artery are measured with the higher of the two values used as the ABI for that leg.
The ABPI test is a popular tool for the non-invasive assessment of Peripheral vascular disease. Studies have shown the sensitivity of ABPI is 90% with a corresponding 98% specificity for detecting hemodynamically significant stenosis >50% in major leg arteries, defined by angiogram.
However, ABPI has known issues:
- ABPI is known to be unreliable on patients with arterial calcification. This is often found in patients with diabetes mellitus, kidney failure or heavy smokers. ABPI values below 0.9 or above 1.3 should be investigated further regardless.
- Resting ABPI is insensitive to mild PAD. Treadmill tests are sometimes used to increase ABPI sensitivity, but this is unsuitable for patients who are obese or have co-morbidities such as Aortic aneurysm, and increases assessment duration.
- Lack of protocol standardisation, which reduces intra-observer reliability.
- Skilled operators are required for consistent, accurate results.
Interpretation of results
In a normal subject the pressure at the ankle is slightly higher than at the elbow.The ABPI is the ratio of the highest ankle to brachial artery pressure. An ABPI between and including 0.90 and 1.29 considered normal, while a lesser than 0.9 indicates arterial disease. An ABPI value of 1.3 or greater is also considered abnormal, and suggests calcification of the walls of the arteries and incompressible vessels, reflecting severe peripheral vascular disease.
Provided that there are no other significant conditions affecting the arteries of the leg, the following ABPI ratios can be used to predict the severity of PAD as well as assess the nature and best management of various types of leg ulcers:
| ABPI value | Interpretation | Action | Nature of ulcers, if present |
| 1.3 and above | Abnormal Vessel hardening from PVD | Refer or measure Toe pressure | Venous ulcer use full compression bandaging |
| 1.0 - 1.2 | Normal range | - | Venous ulcer use full compression bandaging |
| 0.90 - 0.99 | Acceptable | - | Venous ulcer use full compression bandaging |
| 0.80 - 0.89 | Some arterial disease | Manage risk factors | Venous ulcer use full compression bandaging |
| 0.50 - 0.79 | Moderate arterial disease | Routine specialist referral | Mixed ulcers use reduced compression bandaging |
| under 0.50 | Severe arterial disease | Urgent specialist referral | Arterial ulcer no compression bandaging used |