Background A total of 40% of patients with severe aortic stenosis (AS) have low-gradient AS, raising uncertainty about AS severity. Aortic valve calcification, measured by computed tomography (CT), is guideline-endorsed to aid in such cases. The performance of different CT-derived aortic valve areas (AVAs) is less well studied. Methods and Results Consecutive adult patients with presumed moderate and severe AS based on echocardiography (AVA measured by continuity equation on echocardiography <1.5 cm²) who underwent cardiac CT were identified retrospectively. AVAs, measured by direct planimetry on CT (AVA_CT) and by a hybrid approach (AVA measured in a hybrid manner with echocardiography and CT [AVA_Hybrid]), were measured. Sex-specific aortic valve calcification thresholds (≥1200 Agatston units in women and ≥2000 Agatston units in men) were applied to adjudicate severe or nonsevere AS. A total of 215 patients (38.0% women; mean±SD age, 78±8 years) were included: normal flow, 59.5%; and low flow, 40.5%. Among the different thresholds for AVA_CT and AVA_Hybrid, diagnostic performance was the best for AVA_CT <1.2 cm² (sensitivity, 85%; specificity, 26%; and accuracy, 72%), with no significant difference by flow status. The percentage of patients with correctly classified AS severity (correctly classified severe AS+correctly classified moderate AS) was as follows; AVA measured by continuity equation on echocardiography <1.0 cm², 77%; AVA_CT <1.2 cm², 73%; AVA_CT <1.0 cm², 58%; AVA_Hybrid <1.2 cm², 59%; and AVA_Hybrid <1.0 cm², 45%. AVA_CT cut points of 1.52 cm² for normal flow and 1.56 cm² for low flow, provided 95% specificity for excluding severe AS. Conclusions CT-derived AVAs have poor discrimination for AS severity. Using an AVA_CT <1.2-cm² threshold to define severe AS can produce significant error. Larger AVA_CT thresholds improve specificity.

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