Calculated as the ratio of ankle systolic pressure to brachial systolic pressure; a normal ABI is roughly 1.0–1.4, values below 0.9 indicate PAD, and very low values (below 0.4) suggest critical limb ischemia. Notably, ABI can be falsely elevated in heavily calcified, non-compressible vessels (common in diabetics), requiring alternative testing such as toe-brachial index in that scenario.
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