From: Current utility of the ankle-brachial index (ABI) in general practice: implications for its use in cardiovascular disease screening
Aspect of ABI measurement assessed
1. Patient rested in supine position for at least 10 minutes prior to ABI measurement?
• ABI averages 0.35 higher in the seated position as opposed to supine .
• There is no evidence to recommend a minimum period but it should be long enough for blood pressure to return to normal . The effect of the duration of the rest period on the reliability of the ABI measurement is unknown, with most studies using 5-10 minutes .
2. Equipment needed to measure the brachial systolic blood pressure correctly identified as being a Doppler Ultrasound and sphygmomanometer
• Using the Korotkoff method to measure the brachial pressure has been shown to yield lower values compared to Doppler .
• Similarly, automated oscillometric blood pressure devices have been shown to underestimate brachial pressure [12, 13].
• As the brachial pressure forms the denominator of the ABI, underestimation will result in falsely elevated ABIs.
3. Brachial systolic pressure measured in both arms
• A pressure difference between left and right brachial arteries of at least 20 mmHg is present in 3.5% of normal healthy population .
• A recent meta-analysis found that a difference of 15 mmHg or more is actually associated with 2.5 times increased risk of PAD .
• It is therefore paramount that both brachial pressures are measured to prevent missed diagnoses and/or in correct classification of PAD.
4. Equipment needed to measure the ankle systolic blood pressure correctly identified as being a Doppler Ultrasound and sphygmomanometer
• Oscillometric devices have been found to overestimate ankle systolic pressure  resulting in falsely elevated ABIs and reduced sensitivity for detecting PAD [17–19].
• Most oscillometric devices are unable to detect low pressures (<50 mmHg) and hence recording failures are frequent in cases of moderate to severe PAD .
5. More than one pulse assessed at each ankle/foot
• Guidelines differ with regard to which of the three ankle arteries should be assessed, although they all agree that it should be more than one.
• NICE guidance specifies that the arteries assessed should always include the peroneal artery as this may be the only one present in some people, particularly those who are diabetics .
6. ABI calculated by dividing the higher of the ankle systolic blood pressures by the higher of the brachial systolic blood pressures
• Although several authors have argued that utilising the lower ankle systolic pressure as the numerator in the ABI would result in greater sensitivity for the identification of early PAD [20, 21], others have argued that the higher pressure should be used to prevent over diagnosis in healthy subjects .
• Others argue that standardisation of the calculation is the important issue, because this would optimise accuracy and consistency of results universally hence ensuring PAD diagnoses are based on the same parameters [22, 23].