Ankle-Brachial Pressure Index (ABPI) Calculator
Ankle-Brachial Pressure Index (ABPI) Calculator
One leg at a time, with the convention stated: the higher of the two arms underneath, either ankle convention on top, and the trap that a normal or high index in a calcified limb is the dangerous answer.
Ankle-brachial pressure index
One limb, both conventionsDorsalis pedis 118, posterior tibial 104, right brachial 142, left brachial 136 mmHg, higher-ankle convention
Formula
- ankle systolic pressure
- conventionally the HIGHER of the dorsalis pedis and posterior tibial pressures in that leg. The AHA’s 2012 statement, as summarised in American Family Physician, says the index is calculated “by dividing the higher of the posterior tibial or dorsalis pedis blood pressure”. Some protocols and much research use the lower instead
- higher brachial systolic pressure
- “the higher of the right or left arm systolic blood pressure”, per the same statement. Both arms, not one; the higher arm gives the lower, more conservative index
- one index per leg
- the statement is explicit that the index “should be reported separately for each leg”. The denominator is shared between the legs; the numerator is not. A patient has two indices
- why the convention matters
- against duplex in 216 subjects, the abstract of Schroeder and colleagues’ 2006 paper reports the low-ankle method at sensitivity 0.89 and specificity 0.93 and the high-ankle method at 0.68 and 0.99, two-tailed P below .0001 by McNemar’s test. StatPearls records thresholds varying from 0.87 to 0.95 across the variants, and best repeatability with the highest ankle over the highest arm
- measurement conditions, and what it is not
- StatPearls records that sitting rather than lying raises the index by approximately 0.3 and that the minimal significant difference between two readings is 0.15; Wounds UK advises 10 to 15 minutes of rest and a cuff at least 40 per cent of the limb circumference. It is not a resistance: an ankle pressure is a peripheral segmental pressure, and systemic vascular resistance and mean arterial pressure answer a different question
Worked example
Dorsalis pedis 118, posterior tibial 104, right brachial 142, left brachial 136 mmHg, higher-ankle convention
Ankle pressure, higher convention: max(118, 104) = 118 mmHg. Brachial pressure, higher arm: max(142, 136) = 142 mmHg
118 / 142 = 0.83, which is 0.90 or less and therefore abnormal by the ACC/AHA bands
Switch the convention and the same limb reads 104 / 142 = 0.73 — a gap of 0.10, with nothing about the limb changed. Switch the denominator to the left arm's 136 and it reads 0.87
The two directions, separately. Raise the ankle pressure to 130 and the index rises to 0.92, into the borderline band. Raise the higher brachial pressure to 160 and it falls to 0.74. A numerator and denominator that happen to be equal hide a transposition; these two do not
The other leg is a separate calculation. With the same arms and a left dorsalis pedis of 76 and posterior tibial of 84, that leg reads 0.59. One patient, 0.83 and 0.59
The dangerous answer. A diabetic limb with incompressible crural arteries might give a dorsalis pedis of 205: 205 / 142 = 1.44, above the ACC/AHA's 1.40, and the published response is a toe pressure rather than accepting the number
The published bands, and where the sources part company
| ABPI | ACC/AHA (AFP summary) | Other published renderings |
|---|---|---|
| 0.90 or less | Abnormal — peripheral arterial disease | Radiopaedia: 0.4 to 0.9 mild to moderate, under 0.4 severe. GPnotebook draws the line below 0.95. Wounds UK: 0.81 to 0.9 mild, 0.5 to 0.80 moderate, under 0.5 severe, under 0.4 critical |
| 0.91 to 0.99 | Borderline | AHA 2012: 0.91 to 1.00 borderline for cardiovascular risk |
| 1.00 to 1.40 | Normal | ESVS 2024: normal range 0.91 to 1.39 |
| Above 1.40 | Non-compressible — measure a toe-brachial index | ESVS 2024: 1.4 or more is inconclusive. GPnotebook: above 1.3, with local referral above 1.2. Wounds UK: above 1.3. Radiopaedia: about 1.3 to 1.4, no exact cut-off |
What the two ankle conventions do to the same limb
| Convention | Numerator | Reported performance against duplex |
|---|---|---|
| Higher ankle pressure (HAP) | max(dorsalis pedis, posterior tibial) | Sensitivity 0.68, specificity 0.99 — the AHA 2012 method |
| Lower ankle pressure (LAP) | min(dorsalis pedis, posterior tibial) | Sensitivity 0.89, specificity 0.93 |
The falsely normal index, and why the convention comes first
The ankle-brachial pressure index is two cuff pressures divided by one another, and almost everything that goes wrong with it goes wrong before the division. The denominator is the higher of the two brachial pressures, not the right arm by habit; the numerator is conventionally the higher of the dorsalis pedis and posterior tibial pressures in the leg being assessed; and the result belongs to that leg alone. The AHA’s 2012 statement is explicit on all three, including that the index “should be reported separately for each leg”.
The ankle convention is the part sources disagree about, and it is not a quibble. Taking the lower of the two pedal pressures can only lower the index, so it systematically moves limbs towards a diagnosis. Against duplex, the abstract of Schroeder and colleagues’ 2006 paper reports the low-ankle method at sensitivity 0.89 and specificity 0.93 and the guideline high-ankle method at 0.68 and 0.99. StatPearls records that every variant carries prognostic information, with the diagnostic threshold shifting between 0.87 and 0.95 depending on which is used.
Then the trap. A low index means disease; a normal or high index does not mean its absence. Medial arterial calcification — Monckeberg’s sclerosis, classically in long-standing diabetes and in chronic kidney disease — stiffens the crural arteries so that the cuff cannot occlude them at the true intraluminal pressure. The ankle pressure then reads high and the index comes out normal, or above 1.40, in a limb that may be severely ischaemic. That is why NICE instructs clinicians not to exclude peripheral arterial disease in a person with diabetes on a normal or raised index, why the ACC/AHA guideline sends a result above 1.40 to a toe pressure, and why that guideline also says neither index is useful for diagnosing chronic limb-threatening ischaemia, in which at least a quarter of patients have borderline or normal values. An index above the normal range is a finding, not reassurance.
This category owns the limb and the named vessel. Risk estimation in somebody with no vascular diagnosis belongs to the pooled cohort equations, the cost of an operation to the Revised Cardiac Risk Index and the AUB-HAS2 index, and central circulatory arithmetic to systemic vascular resistance and mean arterial pressure.
Frequently asked questions
Which brachial pressure goes in the denominator?
The higher of the two arms. The AHA’s 2012 statement specifies “the higher of the right or left arm systolic blood pressure”, and the same denominator is shared by both legs. An inter-arm difference of 15 to 20 mmHg or more is itself a finding.
Higher or lower of the dorsalis pedis and posterior tibial pressures?
Guidelines say higher; much research uses lower, and the two are not interchangeable. Against duplex in 216 subjects, the abstract of Schroeder and colleagues’ 2006 paper gives the lower-ankle method sensitivity 0.89 and specificity 0.93 and the higher-ankle method 0.68 and 0.99. Because the lower pressure can never exceed the higher, the choice moves results in one direction only. State which you used.
Why is one index per leg not enough?
Because peripheral arterial disease is usually asymmetrical and the legs share only the denominator. The AHA’s statement says the index should be reported separately for each leg; a single number for a patient throws away which leg it is.
What does an ABPI above 1.3 or 1.4 mean?
That the arteries could not be compressed, so the number is not a measurement of perfusion. Which figure marks the boundary depends on the source: ACC/AHA and ESVS 2024 use above 1.40 and 1.4 or more; GPnotebook and Wounds UK use above 1.3; Radiopaedia says about 1.3 to 1.4 without an exact cut-off. The published response is a toe pressure.
Can a normal ABPI exclude peripheral arterial disease?
No, and two guidelines say so in terms. NICE instructs that the diagnosis should not be excluded in a person with diabetes on a normal or raised index. The ACC/AHA guideline states that neither index is useful for diagnosing chronic limb-threatening ischaemia and that at least 25 per cent of those patients have borderline or normal values.
Related calculators
References
- American Academy of Family Physicians. AHA Releases Recommendations on Ankle-Brachial Index Measurement and Interpretation. Am Fam Physician. 2013;88(12):866 — summarising Aboyans V, Criqui MH, Abraham P, et al., Circulation. 2012;126(24):2890–909.
- American Academy of Family Physicians. Management of Lower Extremity Peripheral Artery Disease: Guidelines From the ACC/AHA. Am Fam Physician. 2025;112(5).
- Schröder F, Diehm N, Kareem S, et al. A modified calculation of ankle-brachial pressure index is far more sensitive in the detection of peripheral arterial disease. J Vasc Surg. 2006;44(3):531–6.
- Radiopaedia. Ankle brachial index. Melbourne: Radiopaedia.org, 2025.
- GPnotebook. Ankle brachial pressure index (ABPI). Oxford: GPnotebook, 2025.
- StatPearls. Ankle-Brachial Index. Treasure Island, FL: StatPearls Publishing, 2025.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/