Ankle-brachial index · false normal
Why a normal ankle-brachial index can mislead
A normal ankle-brachial index can be misleading when the ankle arteries are calcified, as they often are in long-standing diabetes and chronic kidney disease. The stiff artery resists the cuff, so the ankle pressure reads falsely high and the ratio looks normal. The toe-brachial index and pulse volume recording are measured instead.
A calcified artery cannot be squeezed. The cuff reads high, the ratio looks fine, and a leg with real disease produces a reassuring number.
How is the ankle-brachial index calculated?
The ankle-brachial index is the highest systolic pressure measured at the ankle divided by the highest measured at the arm. In a healthy circulation the two are close, so the ratio sits near one. When an artery upstream is narrowed, ankle pressure falls and the ratio drops.
It is non-invasive, takes minutes, and is the standard first measurement for lower-limb arterial disease. Its measurement and interpretation are set out in detail in the American Heart Association scientific statement on the subject.
Why can a normal ankle-brachial index be wrong?
The measurement depends on the cuff being able to compress the artery. Medial calcification — calcium in the vessel wall — makes the artery stiff and resistant to compression. The cuff has to be inflated to a higher pressure before flow stops, so the recorded ankle pressure is falsely high and the ratio comes out normal or even elevated.
The populations in which this happens are exactly the populations being screened: long-standing diabetes and chronic kidney disease in particular.
What does a high ankle-brachial index mean?
An index that is unexpectedly high is not a healthier artery. It is usually a stiffer one, and it should be treated as an uninterpretable result rather than a good one.
What test is used when the ankle-brachial index is unreliable?
- The toe-brachial index. The small digital arteries in the toes are much less prone to medial calcification, so a toe measurement stays valid where the ankle one does not.
- Pulse volume recording. The waveform reflects the volume change in the limb with each heartbeat and does not depend on compressing the artery at all. Because Doppler sensitivity is itself reduced by calcification, volume plethysmography is often the practical way to obtain a peripheral waveform in these patients.
- Pulse waveform contour at the toe. Measured optically, and reflecting flow at the far end of the circulation.
How do I interpret my ABI results?
| Index | Waveform | Reading |
|---|---|---|
| Reduced | Damped | Consistent with significant arterial disease |
| Normal or elevated | Damped | Suspect calcification — weight the waveform and the toe index |
| Normal | Normal | Against a significant arterial cause; look elsewhere for the symptom |
| Markedly asymmetric | Asymmetric | Asymmetric disease, clinically meaningful even within range |
The practical consequence
If you have diabetes or kidney disease and have been told your circulation is fine on the basis of an ankle pressure alone, that is worth revisiting — particularly if you have symptoms, a slow-healing sore, or feet that have stopped sweating. Leg symptoms and circulation.
Frequently asked questions
What are normal ABI readings?
In a healthy circulation the pressure at the ankle and the pressure at the arm are close, so the ratio sits near one. When an artery upstream is narrowed, ankle pressure falls and the ratio drops. A reading that looks normal or even high is only reassuring if the artery can be compressed, which is not the case when its wall is calcified.
Can you have peripheral artery disease with a normal ABI?
Yes. Medial calcification, calcium in the artery wall, makes the vessel stiff and resistant to the cuff, so the recorded ankle pressure is falsely high and the ratio comes out normal. This happens most in long-standing diabetes and chronic kidney disease, the same people most often screened. A damped waveform alongside a normal index is the clue.
What is the toe-brachial index?
It compares blood pressure in the toe with pressure in the arm. The small arteries in the toes are much less prone to medial calcification than the ankle arteries, so a toe measurement stays valid where the ankle one does not. It is taken in the same study as the ankle-brachial index and the pulse volume waveform.
References
- Aboyans V, Criqui MH, Abraham P, et al. Measurement and interpretation of the ankle-brachial index: a scientific statement from the American Heart Association. Circulation. 2012;126(24):2890–2909. doi:10.1161/CIR.0b013e318276fbcb
- Fowkes FGR, Rudan D, Rudan I, et al. Comparison of global estimates of prevalence and risk factors for peripheral artery disease in 2000 and 2010: a systematic review and analysis. Lancet. 2013;382(9901):1329–1340. doi:10.1016/S0140-6736(13)61249-0
Related reading
Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, medical director of Measura. Last reviewed .