Ankle-Brachial Index

For physicians

What is measured

Cuff-based oscillometric acquisition with pulse wave velocity, using four limb cuffs plus finger and toe photoplethysmographic sensors. The acquisition yields the ankle-brachial index, the toe-brachial index, an arterial stiffness index and the associated pulse volume waveforms from a single study.

Interpretation

The ankle-brachial index is the ratio of ankle to brachial systolic pressure, validated in adults at risk of or with peripheral arterial disease and addressed in detail in the American Heart Association scientific statement on its measurement and interpretation. Standard acquisition conditions — supine rest, cuff placement and limb positioning — are the dominant source of variance and are enforced procedurally rather than assumed.

The critical interpretive caveat is medial calcification. Incompressible ankle vessels produce falsely normal or elevated indices in limbs with genuine disease, and the affected population — long-standing diabetes, chronic kidney disease — is precisely the screened population. Weight the toe-brachial index and the waveform morphology when the index and the clinical picture disagree. Interpreting the report.

Clinical context

Peripheral artery disease is the third leading cause of atherosclerotic cardiovascular morbidity after coronary disease and stroke. A systematic review of community studies defining disease as an index at or below 0.90 estimated 202 million people affected worldwide in 2010, with prevalence in the wealthier countries surveyed rising from around 5% at ages 45–49 to around 18% at ages 85–89. Meta-analysed associations were strongest for current smoking, then diabetes, hypertension and high cholesterol.

Where it fits

Run alongside pulse volume recording and pulse waveform analysis, and next to sudomotor testing where the question is a foot at risk rather than claudication alone.

Evidence and limitations

Written for a reader who wants the literature rather than the summary. Where the evidence is thin we say so and give both sides, and where a claim is mechanistic rather than demonstrated we label it as such.

Mechanism

A stenosis upstream of the ankle produces a pressure drop across it. Comparing the highest ankle systolic pressure with the highest brachial systolic pressure therefore estimates the hemodynamic significance of disease in between, without imaging anything. The measurement assumes the ankle artery can be occluded by an external cuff.

Medial arterial calcification breaks that assumption. Calcium deposited in the tunica media makes the vessel stiff and, in the extreme, incompressible: the cuff must be inflated well above true intraluminal pressure before flow stops, so the recorded ankle pressure — and therefore the index — is falsely high. The populations in which this occurs are precisely those being screened, since medial arterial calcification is frequently associated with diabetes and chronic kidney disease. The digital arteries of the toes are far less prone to it, which is the physiological basis for the toe-brachial index; pulse volume recording sidesteps the problem entirely by measuring limb volume change rather than occlusion pressure.

What the evidence shows

Peripheral artery disease is the third leading cause of atherosclerotic cardiovascular morbidity after coronary artery disease and stroke. A systematic review of 34 community-based studies including 112,027 participants, of whom 9,347 had peripheral artery disease defined as an ankle-brachial index at or below 0.90, estimated 202 million people living with the disease worldwide in 2010. In the wealthier settings surveyed, prevalence rose from 5.28% (95% CI 3.38 to 8.17) in women and 5.41% (3.41 to 8.49) in men aged 45–49, to 18.38% (11.16 to 28.76) and 18.83% (12.03 to 28.25) respectively by ages 85–89.

Meta-analysed odds ratios in the same review put current smoking first (2.72, 95% CI 2.39 to 3.09 in the wealthier settings), then diabetes (1.88, 1.66 to 2.14), hypertension (1.55, 1.42 to 1.71) and high cholesterol (1.19, 1.07 to 1.33). Measurement technique and interpretation are set out in detail in the American Heart Association scientific statement on the index.

On the calcification problem specifically, a 2022 systematic review of 23 studies of bedside tests in patients prone to medial arterial calcification reached a deliberately cautious conclusion: an index below 0.90 was helpful for diagnosing peripheral artery disease but failed to rule it out, with a negative likelihood ratio above 0.2. Toe pressure and the toe-brachial index performed similarly poorly at ruling disease out. Loss of a triphasic continuous-wave Doppler pattern was reliable for exclusion (negative likelihood ratio 0 to 0.09) but had been validated in only two studies. The authors counseled against relying on any single bedside test.

Limitations of that evidence

• Methodological quality is poor in this literature. The 2022 review judged 20 of 23 included studies to carry a risk of bias, and said so.
• Prevalence estimates rest on the same index they are testing. Defining disease as an index at or below 0.90 means calcified limbs with falsely normal indices are counted as unaffected, which biases prevalence downward in exactly the diabetic and renal populations of most interest.
• The toe-brachial index is not a rescue. It is more robust to calcification than the ankle index and, on the pooled evidence, still poor at ruling disease out.
• Cohorts skew older and male, with a high proportion of diabetes.
• Acquisition conditions — supine rest, cuff size and placement, limb position — are the dominant source of variance and are unevenly standardized.

What remains uncertain

The practical uncertainty is not whether the index measures something real; it is how confidently a normal result can be used to reassure a patient whose arteries may be incompressible. The evidence says: not very. That is the reason this suite acquires the index, the toe-brachial index and the waveform in one sitting and reads them together, and the reason a normal index alongside a damped waveform is treated as calcification rather than as health.

Whether population screening with the index reduces cardiovascular events is a separate and still-contested question, and nothing on this site should be read as claiming it settled. What is well supported is that a measurably abnormal index identifies a patient with systemic atherosclerosis whose conventional risk factors merit aggressive attention.