From the Guidelines
The Ankle Brachial Index (ABI) formula is calculated by dividing the higher systolic ankle pressure of the target leg (posterior tibial or dorsalis pedis artery) by the higher systolic brachial pressure, irrespective of the target leg.
ABI Calculation
- The formula is: ABI = Higher systolic ankle pressure / Higher systolic brachial pressure
- This calculation is used to diagnose and assess the severity of Peripheral Artery Disease (PAD) 1
Interpretation of ABI Values
- Normal ABI: 1.00 to 1.40
- Borderline ABI: 0.91 to 0.99
- Abnormal ABI: 0.90 or less
- Non-compressible arteries: ABI > 1.40 1
Clinical Application
- ABI is a low-cost, easy, and widely used tool for PAD diagnosis and surveillance 1
- An ABI ≤0.90 confirms PAD, with a sensitivity of 68%–84% and a specificity of 84%–99% 1
- ABI results should be uniformly reported, with non-compressible values defined as greater than 1.40, normal values 1.00 to 1.40, borderline 0.91 to 0.99, and abnormal 0.90 or less 1
From the Research
Ankle Brachial Index (ABI) Formula
The Ankle Brachial Index (ABI) is calculated as the ratio of the systolic blood pressure at the ankle to the systolic blood pressure in the upper arm (brachial) [ 2, 3,4,5,6 ].
Calculation Methods
- The ABI can be calculated using the higher of the two ankle pressures (anterior and posterior tibial arteries) in relation to the higher of the left or right brachial systolic blood pressure 4.
- Alternatively, the ABI can be calculated using the lower of the two ankle pressures in relation to the higher of the left or right brachial systolic blood pressure, which may help identify more patients at risk for cardiovascular events 4.
Interpretation of ABI Values
- An ABI value of less than 0.9 indicates peripheral arterial disease (PAD) and is a strong predictor of cardiovascular events 3, 4, 5, 6.
- An ABI value of more than 1.3 may also be indicative of PAD and increased cardiovascular risk, particularly in diabetic patients 5.
- An ABI value of more than 1.4 may be considered pathological, indicating the presence of PAD 6.
Considerations for Specific Populations
- In diabetic patients, the sensitivity of the standard ABI threshold of 0.9 may be lower, and values should be interpreted with precision, taking into account clinical factors such as peripheral neuropathy and arterial calcification 5.