How should I measure the ankle‑brachial index at bedside in an older patient with type 2 diabetes mellitus, hypertension, hyperlipidemia, chronic limb‑threatening ischemia and a smoking history, given the risk of non‑compressible tibial arteries and the possible need for toe‑brachial index confirmation?

How to Measure Ankle-Brachial Index (ABI) in High-Risk Diabetic Patients

In this older patient with diabetes, hypertension, hyperlipidemia, chronic limb-threatening ischemia and smoking history, measure bilateral brachial pressures and bilateral ankle pressures (both dorsalis pedis and posterior tibial arteries) using a handheld Doppler device, but immediately proceed to toe-brachial index (TBI) measurement regardless of the ABI result, because arterial calcification from diabetes renders ABI unreliable in this population. 1, 2

Step-by-Step ABI Measurement Protocol

Patient Preparation and Equipment

• Position the patient supine for 10 minutes before measurement to allow hemodynamic stabilization 1
• Use a handheld continuous-wave Doppler device with appropriate gel and a standard blood pressure cuff 1
• Ensure the room is warm to prevent arterial vasoconstriction that could produce falsely low readings 3

Measurement Sequence

• Measure brachial pressures first: Apply the blood pressure cuff to the upper arm and locate the brachial artery with the Doppler probe at a 45° to 60° angle to the skin surface 1
• Inflate the cuff progressively to 20 mmHg above the level where the flow signal disappears (maximum 300 mmHg), then deflate slowly to detect the pressure at which flow signal reappears 1
• Measure both arms: Use a consistent sequence (e.g., right arm → right posterior tibial → right dorsalis pedis → left posterior tibial → left dorsalis pedis → left arm) 1
• Repeat the first arm measurement at the end of the sequence and average both results unless they differ by >10 mmHg, in which case use only the second measurement 1

Ankle Pressure Measurement

• Apply the blood pressure cuff just above the ankle 1
• Locate both the dorsalis pedis and posterior tibial arteries with the Doppler probe using the same 45° to 60° angle 1
• Measure systolic pressure at both arteries on each leg using the same inflation/deflation technique 1
• Record Doppler waveforms at each ankle artery—triphasic waveforms strongly exclude PAD, while monophasic or absent waveforms suggest significant disease 1, 2

ABI Calculation

• Use the higher of the two brachial pressures as the denominator 1
• For the numerator, use the higher ankle pressure (dorsalis pedis or posterior tibial) from each leg to maximize specificity and minimize false-positive diagnoses 1
• Calculate ABI for each leg separately: ABI = (higher ankle pressure on that leg) ÷ (higher brachial pressure) 1

Critical Interpretation in This Diabetic Patient

ABI Values and Their Meaning

• ABI ≤0.90: Confirms PAD 1
• ABI 0.91-0.99: Borderline, warrants additional testing 1
• ABI 1.00-1.40: Traditionally "normal" but unreliable in diabetes—proceed to TBI regardless 1, 2, 4
• ABI >1.40: Indicates non-compressible vessels from medial arterial calcification (Mönckeberg sclerosis)—ABI is invalid, TBI is mandatory 1, 2, 4

Why ABI Alone Is Insufficient in This Patient

• Medial arterial calcification is highly prevalent in diabetic patients and causes falsely elevated or normal ABI readings despite significant stenotic disease 1, 2, 4
• Up to 50% of patients with ABI >1.40 have coexisting PAD 2
• In patients with chronic limb-threatening ischemia, 29% have ABI between 0.70-1.40, and ABI may be normal in nearly one-quarter of cases 3
• The concordance between ABI and toe pressure/TBI in chronic limb-threatening ischemia is poor, with only 58% of patients meeting criteria for abnormal toe pressures presenting with abnormal ABIs 3
• Sensitivity of ABI is significantly lower in diabetic patients with complications, particularly when neuropathy or chronic kidney disease coexist 2, 4, 5

Mandatory Toe-Brachial Index (TBI) Measurement

When to Measure TBI

• Immediately in this patient, regardless of ABI result, because diabetes, chronic limb-threatening ischemia, and the risk of non-compressible arteries make ABI unreliable 1, 2, 3
• TBI should be performed because digital arteries are rarely affected by medial calcification 2

TBI Measurement Technique

• Use a small blood pressure cuff placed around the base of the great toe 1
• Measure toe systolic pressure using photoplethysmography or Doppler 1
• Calculate TBI = (toe systolic pressure) ÷ (higher brachial pressure) 1

TBI Interpretation

• TBI <0.70: Confirms PAD 1, 2
• TBI <0.75: Suggests significant PAD (alternative threshold) 2
• TBI ≥0.75: Largely excludes PAD 2
• Abnormal toe pressures reflect severe ischemia and are associated with decreased likelihood of wound healing 3

Additional Vascular Assessment in Chronic Limb-Threatening Ischemia

Supplementary Tests to Consider

• Transcutaneous oxygen pressure (TcPO2): Values >30 mmHg predict wound healing potential 1, 3
• Skin perfusion pressure (SPP): Values >40 mmHg predict wound healing potential 1, 3
• Pulse volume recordings (PVR): Can confirm concordance with ABI findings and suggest non-compressible arteries when discordant 1
• Segmental pressures with waveforms: Useful when anatomic localization is required to plan revascularization 1

Common Pitfalls to Avoid

Critical Errors in This Clinical Context

• Never rely on ABI alone in diabetic patients—always obtain TBI or waveform analysis because arterial calcification causes falsely elevated readings 1, 2, 4
• Never assume PAD is absent based solely on palpable pulses—even skilled examiners can detect pulses despite significant ischemia 2, 6
• Never delay vascular assessment—clinical examination sensitivity is too low to rule out PAD, and objective testing is mandatory in all patients with chronic limb-threatening ischemia 2
• Never use only one ankle artery pressure—measure both dorsalis pedis and posterior tibial arteries because atherosclerotic lesions in diabetes predominantly reside in below-the-knee arteries that lie parallel to each other, making pressure in one artery less representative of overall disease 5
• Never debride ischemic heel ulcers without infection until vascular status is fully assessed and revascularization considered 2

Technical Considerations

• If arterial flow is not detected at the ankle and the ABI based on the other ankle artery is normal, perform duplex ultrasound to determine whether PAD is present rather than assuming arterial agenesis 1
• Avoid measuring ABI in a cold room, as vasoconstriction can produce inaccurate results 3
• When repeat measurements are needed, perform them in reverse order of the initial sequence to minimize systematic bias 1

Immediate Next Steps Based on Results

If TBI Confirms PAD (TBI <0.70-0.75)

• Strongly consider revascularization when ankle pressure <50 mmHg or ABI <0.5, or when patent posterior tibial artery to the ankle is critical for heel perfusion 2
• Proceed to duplex ultrasound, CT angiography, or magnetic resonance angiography to assess anatomy and determine revascularization strategy 1
• Implement aggressive cardiovascular risk reduction: smoking cessation support, statin therapy, low-dose aspirin or clopidogrel, blood pressure control, and glycemic control 2

If Results Are Equivocal or Discordant

• Obtain TcPO2 and/or SPP measurements to determine likelihood of wound healing without or after revascularization 1, 3
• Consider vascular surgery referral for comprehensive evaluation and potential catheter angiography 1