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Assessing limb perfusion is best done with a simple, objective measure of arterial blood flow to the leg. The Ankle-Brachial Index does this by comparing the systolic blood pressure at the ankle with the systolic pressure in the arm. This ratio reflects how well blood is reaching the lower limb. A normal value is around 1.0, and lower values indicate increasing arterial insufficiency and higher risk of limb ischemia, guiding urgent evaluation or intervention.

SpO2 looks at oxygen saturation in the blood, not how much blood is actually reaching the limb tissues, so it can miss perfusion problems. CT angiography provides anatomical detail of vessels and exposes the patient to contrast and radiation, and isn’t a quick bedside perfusion measure. Doppler ultrasound can assess flow patterns but doesn’t give the simple, standardized perfusion index that ABI provides.

So the Ankle-Brachial Index is the most appropriate test to assess limb perfusion in this context.