Nursing care
Ankle-Brachial Index: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
The ankle-brachial index compares systolic blood pressure at the ankle to the arm to screen for peripheral arterial disease. A ratio below 0.9 indicates PAD, while a ratio above 1.3 suggests calcified, non-compressible vessels rather than healthy circulation — common in diabetics — which makes the number unreliable and prompts a different test instead.
Why this value is ordered
The ankle-brachial index is ordered when a patient presents with intermittent claudication, non-healing lower-extremity wounds, absent or diminished pedal pulses, or risk factors for peripheral arterial disease such as diabetes, smoking history, or long-standing hypertension. It is a simple bedside comparison: systolic pressure at the ankle divided by systolic pressure at the arm, using a handheld Doppler.
It is also used before procedures that depend on adequate lower-limb perfusion, such as wound debridement or compression therapy, because applying compression to a limb with significant arterial insufficiency can worsen ischaemia rather than help venous return.
Interpreting the number in context
A normal ABI sits between 1.0 and 1.3. Below 0.9 confirms peripheral arterial disease, with lower numbers corresponding to more severe disease — 0.5 to 0.9 suggests moderate disease, and below 0.5 suggests severe, limb-threatening ischaemia.
An ABI above 1.3 does not mean healthy vessels. It means the ankle vessels are calcified and cannot be compressed by the blood pressure cuff, so the reading is falsely elevated and clinically meaningless. This pattern is common in patients with diabetes or chronic kidney disease, both of which promote medial arterial calcification. A result above 1.3 should prompt a different diagnostic approach, not reassurance.
Critical values and what to do
An ABI below 0.5, especially alongside rest pain, pallor, or a cool, pulseless limb, signals critical limb ischaemia and needs urgent vascular surgery referral. This is a limb-threatening finding, not a routine outpatient result.
Report any ABI below 0.9 to the provider along with the clinical picture, since management ranges from lifestyle modification and antiplatelet therapy for mild disease to angiography or surgical revascularisation for severe disease. A falsely elevated ABI above 1.3 in a symptomatic patient should also be reported, because it means the test has failed to answer the clinical question and further workup is needed.
Related tests read alongside it
When the ABI is unreliable due to calcification, the toe-brachial index is used instead, since digital vessels calcify less readily and give a more accurate picture in diabetic patients. Duplex ultrasound and CT or MR angiography provide direct visualisation of stenosis and occlusion when more detail is needed.
Pulse volume recordings and segmental pressures can localise the level of disease along the limb, which matters for surgical planning. None of these replace a careful pulse check and skin assessment, which remain part of the same clinical picture.
Nursing implications
Position the patient supine and let them rest quietly for several minutes before measurement, since recent activity or a cold room can alter readings. Use the correct cuff size and take pressures at both arms and both ankles, since a significant difference between limbs is itself a finding worth reporting.
Document pulses, capillary refill, skin temperature, and any wounds alongside the ABI value, because the number is only useful read together with the clinical exam. Do not apply compression therapy to a limb with an ABI below 0.5 to 0.8 without explicit provider direction, as compression can precipitate acute ischaemia in significant arterial disease.
What patients ask about it
Patients commonly ask whether the test is painful. It is not — it uses the same blood pressure cuff and Doppler probe used elsewhere, just applied at the ankle as well as the arm, and takes only a few extra minutes.
Patients with diabetes often ask why their result came back normal or high despite obvious symptoms. Explaining that calcified vessels can produce a falsely reassuring number, and that a toe-brachial index gives a more accurate answer, helps them understand why further testing is being ordered rather than the issue being dismissed.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our cardiovascular practice questions are the closest set to what this page covers.
Common questions
What ABI value confirms peripheral arterial disease?
An ABI below 0.9 confirms PAD. Between 0.5 and 0.9 suggests moderate disease, and below 0.5 suggests severe disease with a risk of critical limb ischaemia.
Why is an ABI above 1.3 not a good result?
A value above 1.3 usually means the ankle vessels are calcified and cannot be compressed properly by the cuff, giving a falsely high reading rather than reflecting healthy circulation. This is common in diabetic patients, and a toe-brachial index or imaging study is used instead.
Can a nurse perform an ABI independently?
Yes, ABI measurement is within nursing scope in many settings using a handheld Doppler and blood pressure cuff, though facility policy and state scope-of-practice rules vary. Some institutions restrict it to vascular-trained staff, so check local policy before performing it unsupervised.
Is it safe to apply compression stockings if the ABI is low?
Not without provider guidance. Compression is generally avoided or used with caution when the ABI is below about 0.8, since it can further reduce perfusion in a limb that already has compromised arterial flow.
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