Why a normal ankle-brachial index can mislead
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 the index works
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.
Where it breaks
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.
The rule of thumb
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 is measured instead
- 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.
What the combinations mean
| 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.
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 .