Ankle-Brachial Index (ABI)
Definition
The ankle-brachial index (ABI) is the ratio of the higher systolic pressure in the ipsilateral dorsalis pedis or posterior tibial artery divided by the higher of the left or right brachial artery systolic pressure. It is the cornerstone noninvasive test for diagnosis of entities/Peripheral-Artery-Disease, performable with a blood pressure cuff and a handheld Doppler device in a vascular laboratory or office setting. sources/PVD-AHA-2024
Key Concepts
ABI Classification (Standardised Reporting)
| ABI Value | Interpretation |
|---|---|
| ≤0.90 | Abnormal — PAD confirmed |
| 0.91–0.99 | Borderline — possible PAD |
| 1.00–1.40 | Normal |
| >1.40 | Noncompressible — ABI unreliable; use TBI |
Standardised reporting allows within-patient and between-facility comparison. COR 1 B-NR for symptomatic patients. sources/PVD-AHA-2024
Diagnostic Performance
- Sensitivity 69–79%, specificity 83–99% for clinically significant arterial stenosis compared with imaging
- Reduced sensitivity in diabetes (calcified vessels)
- Continuous-wave Doppler waveforms or PVR (pulse volume recordings) at the ankle supplement the ABI for concordance confirmation sources/PVD-AHA-2024
Indications
- COR 1: Patients with history or physical examination findings suggestive of PAD (see PAD clinical presentation)
- COR 2a: Screening in high-risk patients (age ≥65; age 50–64 with risk factors; age <50 with diabetes + risk factor; known polyvascular disease)
- COR 3:No Benefit: Screening in low-risk patients without symptoms or signs of PAD sources/PVD-AHA-2024
Limitations and Special Situations
Noncompressible vessels (ABI >1.40):
- Common in diabetes and CKD due to medial arterial calcification; falsely elevated ABI despite significant PAD
- Digital arteries are rarely noncompressible → TBI (Toe-Brachial Index) is the alternative test
- TBI ≤0.70 is abnormal and establishes PAD diagnosis
- Absolute toe pressure <30 mmHg = severe ischaemia; associated with major amputation and poor wound healing
- Photoplethysmographic waveform from the great toe base further supports PAD diagnosis in suspected CLTI sources/PVD-AHA-2024
CLTI assessment:
- ABI may be in the normal range (0.90–1.40) in nearly 25% of CLTI patients; concordance between ABI and toe pressure is poor (only 58% of patients with abnormal toe pressures have an abnormal ABI)
- Additional perfusion measures needed: TBI, TcPO₂ (transcutaneous oxygen pressure), SPP (skin perfusion pressure)
- TcPO₂ >30 mmHg predicts wound healing
- SPP >40 mmHg predicts wound healing
- Perfusion measurements must be obtained in a warm room to prevent cold-induced vasoconstriction sources/PVD-AHA-2024
Exercise Treadmill ABI
- COR 1 B-NR: Indicated when resting ABI is normal or borderline (>0.90 to ≤1.40) but exertional leg symptoms suggest PAD
- COR 2a B-NR: Useful to objectively assess functional status and walking performance when resting ABI ≤0.90
- Performed within 1–5 minutes after exercise on motorised treadmill; increases blood flow gradient across stenoses, improving diagnostic sensitivity
- Provides baseline functional data (time to symptom onset, maximal walking time, post-exercise ABI) for tracking response to therapy sources/PVD-AHA-2024
Segmental Leg Pressures and Waveforms
- COR 2a C-LD: Segmental limb pressures with PVR and/or Doppler waveforms to localise anatomic level of PAD (aortoiliac vs femoropopliteal vs infrapopliteal)
- Three or four cuffs placed from thigh to ankle; pressure gradients and waveform changes define the level
- Particularly relevant when considering revascularisation sources/PVD-AHA-2024
ABI as a Risk Marker
- Patients with abnormal ABI and no leg symptoms (asymptomatic PAD) have:
- Poorer cardiovascular morbidity and mortality than normal ABI
- Poorer functional status and more rapid functional decline
- High prevalence of concomitant CAD
- A 1-unit decline in ABI is associated with incremental cardiovascular risk sources/PVD-AHA-2024
Contradictions / Open Questions
- Optimal ABI threshold for diagnosis: Studies use varying ABI thresholds for enrolment (e.g., AAA trial used <0.95; POPADAD used <1.00), with mean enrolled ABIs around 0.86–0.90. This limits extrapolation of treatment trial results to the true abnormal ABI (≤0.90) population. sources/PVD-AHA-2024
- ABI screening programs and mortality benefit: The Danish screening RCT showed a small but significant mortality reduction when ABI screening was combined with hypertension and aneurysm screening; a more extensive screening program in a separate Danish RCT did not show mortality benefit. The independent contribution of ABI screening to the benefit is unclear. sources/PVD-AHA-2024
Connections
- Related to entities/Peripheral-Artery-Disease
- Related to concepts/CLTI
- Related to concepts/Acute-Limb-Ischemia