Introduction: Despite a growing understanding of the role played by plaque morphology, the degree of carotid lumen reduction remains the principle parameter in decisions on revascularization in symptomatic and asymptomatic patients. Computed tomography angiography (CTA) is a widely used guideline-approved imaging modality, with “percent stenosis” commonly calculated as %area reduction (area stenosis – AS). Aim: We evaluated the impact of the non-linear relationship between diameter stenosis (DS) and AS (area = π • (diameter/2)2, so that in concentric lesions 51%AS is 30%DS and 75%AS is 50%DS) on stenosis severity misclassification using calculation of area reduction. Material and methods: CTA and catheter quantitative angiography (cQA) were performed in 300 consecutive patients referred to a tertiary vascular centre for potential carotid revascularization (age: 47–83 years, 33.7% symptomatic, 36% female; referral stenosis of ≥ “50%”). CTA-AS was determined by agreement of 2 experienced radiologists; cQA-DS (pivotal trials standard reference, NASCET method) was calculated by agreement of 2 corelab analysts. Results: For symptomatic lesion thresholds, CTA-AS-based calculation reclassified 76% of “< 50%” cQA-DS measurements to the “50–69%” group, and 58% of “50–69%” measurements to the “≥ 70%” group. For asymptomatic lesion thresholds, 78% of “< 60%” cQA-DS measurements were reclassified to the “60–79%” group, whereas 42% of “60–79%” cQA measurements crossed to the “≥ 80%” class. Overall, employing CTA-AS instead of cQA-DS enlarged the “60–79%” and “≥ 80%” lesion severity classes 1.6- and 5.8-fold, respectively, whereas the “≥ 70%” class increased 4.15-fold. Conclusions: Replacing the pivotal carotid trials reference standard cQA-DS “%stenosis” measurement with CTA-AS-based “%stenosis” results in a large-scale lesion/patient erroneous gain of an “indication” to revascularization or migration to a higher revascularization indication class. In consequence, unnecessary carotid procedures may be performed in the absence of cQA verification. Until guidelines rectify the “%stenosis” measurement methods with different guideline-approved imaging modalities (and, where needed, re-adjust decision thresholds), CTA-AS measurement should not be used as a basis for carotid revascularization.