01Purpose and principlesWhat the assessment is for and the core concepts behind it.
ABPI compares ankle systolic pressure with brachial systolic pressure to detect a pressure loss across the lower-limb arterial circulation. It is a physiological test, not an angiogram. Its value depends on correct preparation, cuff size, Doppler technique, identification of both ankle arteries and transparent calculation. A number without the component pressures and signal quality can hide technical error or incompressible vessels.
Waveforms add information about pulsatility and damping. A normal peripheral continuous-wave Doppler signal usually has brisk systolic upstroke and multiphasic components, while progressive disease may reduce reverse flow, delay the upstroke and produce a low-amplitude monophasic pattern. Terminology and device outputs vary, so waveform morphology should be described and interpreted with symptoms, pulses, pressures and the validated vascular-laboratory protocol. Neither a single threshold nor a normal waveform guarantees tissue healing.
Key points
- Rest the patient supine and measure both arms and both ankle arteries.
- For each leg, divide the highest ankle pressure by the highest arm pressure.
- Record Doppler signal character and technical limitations alongside the numeric index.
- A reduced ABPI supports PAD but does not alone define symptoms or anatomy.
- Normal or raised ABPI cannot exclude PAD in diabetes or incompressible arteries.
- Toe pressures, pulse-volume recordings or exercise testing can resolve selected discordant cases.
- Monophasic damped waveforms support haemodynamically important proximal arterial disease.
- Urgent clinical threat overrides a reassuring or technically limited measurement.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Use ABPI when history or examination suggests PAD, including exertional symptoms, non-healing wounds, unexplained leg pain or previous lower-limb arterial intervention.
The patient rests supine in a warm environment, appropriate cuffs are applied to bare skin, and recent exertion or cold-induced vasoconstriction is avoided where possible.
The cuff cannot abolish the Doppler signal, an unexpectedly high pressure is obtained, or diabetes and kidney disease make medial arterial calcification likely.
Rest pain, ulceration, gangrene or marked waveform abnormality remains concerning even when the calculated ankle index appears normal or raised.
Sudden pain, pallor, coldness, pulse loss, paraesthesia or paralysis requires emergency assessment and should not wait for a complete outpatient protocol.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Manual Doppler ABPIFirst step - Why
- Quantify resting ankle-to-arm systolic pressure for each lower limb.
- Interpretation and limitations
- Record systolic pressure in both arms and the posterior tibial and dorsalis pedis arteries of each ankle. NICE uses the highest ankle pressure in that leg divided by the highest arm pressure. Values below the laboratory normal range support PAD, but borderline, normal or high results require context; many services use around 0.90 as the lower limit while avoiding threshold-only diagnosis.
- 02
Continuous-wave Doppler waveform assessment - Why
- Describe arterial flow quality when pressure alone is incomplete or misleading.
- Interpretation and limitations
- Assess upstroke, amplitude, forward and reverse components and symmetry at named arteries. A damped monophasic signal suggests haemodynamically important disease proximal to the sample; arrhythmia, low output, probe angle and motion can alter appearance, so compare levels and the contralateral side.
- 03
Toe systolic pressure and toe-brachial index - Why
- Estimate distal perfusion when ankle arteries are poorly compressible or wounds raise concern.
- Interpretation and limitations
- Digital arteries are often less affected by medial calcification than tibial arteries. Low toe pressure supports impaired perfusion, but temperature, vasospasm, oedema and digital disease affect the reading; use service-specific thresholds and integrate wound and waveform findings.
- 04
Pulse-volume recording - Why
- Assess segmental volume change without depending on arterial compressibility at one ankle site.
- Interpretation and limitations
- A normal tracing has a brisk upstroke and pulsatile contour; delayed rise, reduced amplitude and loss of features support inflow disease. Cuff placement, oedema and movement cause artefact, and localisation improves when several limb levels are compared.
- 05
Post-exercise ankle pressure and waveform testing - Why
- Reveal exertional flow limitation when resting results do not explain reproducible symptoms.
- Interpretation and limitations
- Use a standardised treadmill or alternative exercise protocol with prompt repeat measurements. A reproducible post-exercise fall supports PAD, but exercise dose and criteria vary by accredited laboratory and the test is inappropriate when walking is unsafe or limb threat is present.
- 06
Duplex arterial ultrasound - Why
- Localise disease and assess lesion haemodynamics when anatomy will change management.
- Interpretation and limitations
- Duplex combines vessel imaging with velocity and waveform change. NICE selects duplex first when revascularisation is being considered; further contrast-enhanced MRA or CTA depends on whether more detail is needed and whether MRA is tolerated.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked exampleCalculate and challenge a discordant ABPIFirst stepA person with diabetes has exertional calf pain and a technically high ankle pressure.+
- 1Inputs: right arm 142 mmHg, left arm 136 mmHg, right dorsalis pedis 168 mmHg and right posterior tibial 174 mmHg, with a damped monophasic pedal signal.
- 2Calculation and reasoning: 174 divided by 142 gives 1.23, but diabetes, unexpectedly high ankle pressure and a damped monophasic waveform make incompressibility and occult PAD plausible.
- 3Reasoning: the index is not low, but diabetes, unexpectedly high ankle pressures and a monophasic waveform make incompressibility and occult PAD plausible.
- 4Action: repeat technical checks and obtain toe pressure or vascular-laboratory assessment; use duplex if revascularisation is being considered or the anatomy will change care.
- 5Verification: document every component pressure, cuff and signal limitation, then reconcile the added physiological result with symptoms, pulses and skin rather than declaring PAD excluded.
02Routine acquisitionProduce an auditable bedside indexStable symptoms or examination findings justify resting physiological assessment for PAD.+
- 1Confirm identity, indication, consent, limb wounds and whether compression or positioning will be painful or unsafe.
- 2Allow supine rest, select cuffs of suitable width and obtain both brachial systolic pressures with Doppler.
- 3Measure dorsalis pedis and posterior tibial pressures in each leg, retaining the vessel identity and waveform description.
- 4Calculate each leg separately using its highest ankle pressure and the highest pressure from either arm, then report limitations.
03Discordant resultEscalate uncertainty according to consequenceEscalationThe index conflicts with symptoms, skin findings, pulses, waveform morphology or wound behaviour.+
- 1Recheck rest, cuff size, probe position, vessel identity, device calibration and whether the ankle artery was actually compressible.
- 2Add toe pressure, pulse-volume recording or standardised exercise testing according to the clinical question and service capability.
- 3Seek urgent vascular assessment for tissue threat, and reserve anatomical imaging for a decision likely to alter treatment.
04Post-intervention comparisonUse trends without ignoring the patientA previous revascularisation has a defined surveillance plan or symptoms have materially changed.+
- 1Compare the current index, component pressures and waveforms with measurements made using the same validated method.
- 2Investigate a meaningful deterioration together with recurrent symptoms, pulse change or wound failure rather than relying on numerical noise.
- 3Follow the vascular service surveillance schedule and use duplex when graft or treated-segment anatomy needs direct assessment.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Store both arm pressures, both named ankle-artery pressures, indices, waveforms and all technical limitations.
- Trend results only when acquisition method, position, cuff size and clinical state are sufficiently comparable.
- Reassess promptly when walking symptoms progress, rest pain appears or skin breakdown fails to heal.
- Escalate discordant results for vascular-laboratory testing rather than repeatedly accepting an implausible bedside number.
- After revascularisation, use the specialist surveillance plan and report new symptoms between scheduled tests.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
The denominator matters
Using the lower arm pressure can falsely raise both limb indices; NICE specifies the highest arm pressure, while a substantial inter-arm difference itself deserves clinical assessment.
Preserve raw measurements
Component pressures reveal which artery was used, whether one vessel was absent and whether a recalculation or trend comparison is valid.
Waveforms are contextual
Monophasic damping supports arterial disease but does not by itself identify the exact lesion; compare sequential sites and use imaging when anatomy will alter management.
High is not reassuring
An unusually high ankle pressure may mean cuff incompressibility from medial calcification, especially in diabetes or kidney disease, rather than excellent perfusion.
Healing is multifactorial
Toe or ankle pressure contributes to perfusion assessment, while infection, oedema, nutrition, neuropathy and wound mechanics also affect whether tissue heals.
07Common pitfallsFrequent interpretation and management errors.
- 01
Using one brachial pressure or the lower arm pressure instead of measuring both and selecting the highest.
- 02
Measuring only one ankle artery and concealing which pressure entered the calculation.
- 03
Reporting a ratio without raw pressures, waveforms, patient position or technical limitations.
- 04
Declaring PAD excluded by a normal or raised index in diabetes or suspected calcification.
- 05
Assigning a rigid anatomical stenosis from a waveform without duplex or other appropriate imaging.
- 06
Repeating physiological tests while rest pain, tissue loss or acute neurological deficit requires urgent vascular action.