A 55-year-old male presents to the emergency department with epigastric pain 12 hours after alcohol consumption, followed by vomiting fresh red blood (hematemesis) of 200ml. Physical examination reveals a soft abdomen, tenderness in the upper abdomen, non-palpable liver, spleen palpable 1.5 cm below the costal margin, and normal blood pressure (BP). Which test should be performed first to confirm the diagnosis?