A 32-year-old male presents with irregular fever for 2 months, accompanied by poor appetite, weight loss, and pruritus. Physical examination reveals temperature 39°C, multiple bean-sized hard, mobile, non-tender lymph nodes in the neck and supraclavicular region, liver palpable 1 cm below the costal margin, and spleen palpable 3 cm below the costal margin. Laboratory tests show WBC 25×10<sup>9</sup>/L (N 85%, L 10%, E 5%). Multiple antibiotic treatments were ineffective, but indomethacin provided temporary fever relief. Which test is most appropriate for definitive diagnosis?