A 35-year-old woman presents with fever and night sweats for over 20 days. Temperature is around 38°C. Physical exam reveals a distended abdomen with doughy consistency, mild tenderness and rebound tenderness throughout. Liver and spleen are not palpable below the costal margin. Shifting dullness is positive. WBC count is 7.2*10<sup>9</sup>/L (neutrophils 75%, lymphocytes 25%). If ascitic fluid analysis shows exudate, which test would be most simple and valuable for diagnosis?