Hemodynamic severity is best characterized by the transaortic maximum velocity (or mean pressure gradi- ent) when the transaortic volume flow rate is normal. Some patients with AS have a low transaortic volume flow rate that is either because of LV systolic dysfunc- tion with a low LVEF or because of a small, hypertro- phied LV with a low stroke volume. Severe AS with low flow is designated D2 (with a low LVEF) or D3 (with a normal LVEF). Meticulous attention to detail is required during assessment of aortic valve hemodynamics, either with Doppler echocardiography or cardiac catheteriza- tion, and the inherent variability of the measurements and calculations should always be considered in clinical decision-making. 3.2. Aortic Stenosis 3.2.1. Diagnosis and Follow-Up 3.2.1.1. Diagnostic Testing: Initial Diagnosis Recommendations for Diagnostic Testing: Initial Diagnosis of AS Referenced studies that support the recommendations are summarized in Online Data Supplement 3. 