A 35-year-old woman presents with fever and night sweats for over 20 days. Her temperature is around 38°C. Physical examination reveals a distended abdomen with doughy consistency, mild tenderness and rebound tenderness throughout the abdomen. The liver and spleen are not palpable below the costal margin. Shifting dullness is positive. Laboratory tests show a white blood cell (WBC) count of 7.2×10⁹/L, with 75% neutrophils and 25% lymphocytes. Which test should be prioritized to confirm the diagnosis?