The Ankle-Brachial Index (ABI) has long been a key parameter in diagnosing peripheral arterial disease (PAD). However, in clinical practice, a “normal” ABI does not always rule out circulatory disorders. Especially in early-stage risk patients – such as those with diabetes, medial arterial calcification (MAC), or hypertension – ABI results can be falsely negative. In such cases, additional parameters like the Toe-Brachial Index (TBI) or the Pulse Wave Index (PWI™) become crucial.
The “Normal” ABI – and Why It Can Be Misleading
ABI is calculated by comparing blood pressure at the ankle and the arm. Values between 0.9 and 1.3 are considered normal. However, in patients with stiffened arteries – due to MAC or high blood pressure – ankle pressures may appear falsely elevated. This can result in a normal ABI even when blood flow is compromised.
This limitation is particularly relevant for older patients, those with diabetes, or individuals with chronic kidney disease. In such cases, it’s worthwhile to take a closer look using more sensitive diagnostic parameters.
TBI – A More Sensitive Indicator
The Toe-Brachial Index (TBI) works similarly to ABI but measures blood pressure at the great toe instead of the ankle. Since toe arteries are less affected by MAC, TBI can provide accurate results even in patients with stiffened vessels.
A TBI value below 0.7 is considered pathological and indicates PAD – even if the ABI remains within normal range. TBI is especially useful in detecting early or advanced vascular disease in diabetic patients.
PWI™ – A Modern Alternative to Pressure-Based Diagnosis
The Pulse Wave Index (PWI™) is an innovative, blood pressure–independent parameter for assessing arterial circulation. Rather than comparing absolute pressure values, PWI™ analyzes the shape and propagation of the pulse wave – especially rise time and amplitude.
The advantage: PWI™ is highly sensitive to post-stenotic flow changes and less affected by arterial stiffness or hypertension. This makes it a powerful tool for identifying vascular changes where ABI fails.
Why Additional Parameters Like TBI & PWI™ Are Critical
For patients with a normal ABI but clinical suspicion of PAD, complementary measurements like PWI™ are invaluable. By analyzing the dynamics of pulse waves, PWI™ can detect flow impairments even when ABI readings appear unremarkable – such as in patients with MAC.
A study by Mayr et al. (2019) highlights this advantage:
“In our study, the Pulse Wave Index (PWI) outperformed both oscillometric and Doppler-based ABI in detecting PAD. This is not surprising, as PWI is strongly influenced by the time to peak of the volume curve – a parameter particularly sensitive to post-stenotic flow changes and less affected by arterial stiffness.” (Mayr M. et al., VASA 2019)
For high-risk patients such as those with diabetes, PWI™ provides an added layer of diagnostic confidence – especially in the context of prevention and early risk detection.
Why Differentiated Diagnostics Matter
A normal ABI may offer false reassurance – with potentially serious consequences. Undiagnosed circulatory disorders can lead to poor wound healing, infections, and, in the worst case, amputations. Using additional parameters like TBI and PWI™ increases diagnostic sensitivity and enables targeted preventive measures.
Modern diagnostic systems – such as the AngE™ systems from SOT Medical Systems – combine ABI, TBI, PWI™, and other parameters into a fast, delegable examination workflow. This allows trained medical staff to identify at-risk patients early, even without a physician present.
Conclusion: See More, Treat Better
The Ankle-Brachial Index is a valuable tool – but it has its limitations. In high-risk populations, expanded diagnostics using TBI or PWI™ should become standard practice. Only then can circulatory disorders be reliably detected, and preventive care initiated in time.
SOT Medical Systems supports healthcare professionals with innovative, blood pressure–independent diagnostic solutions that make vascular diagnostics more efficient and reliable.
📩 Interested in learning more about PWI™, TBI, and modern PAD screening? Get in touch with us for a consultation.
Source:
Mayr, M. et al. (2019): “Oscillometric Pulse Wave Index (PWI) – a novel tool to detect peripheral arterial disease in patients with diabetes mellitus”. In: VASA – European Journal of Vascular Medicine. DOI: 10.1024/0301-1526/a000798: https://www.sot-medical.com/download/diagnostic-accuracy-of-abi-by-doppler-versus-4-point-oscillometry/
