A 28-year-old male presents with 5 days of fever accompanied by poor appetite, fatigue, abdominal pain, distension, and diarrhea. He self-medicated with antipyretics, co-trimoxazole, and chloramphenicol, but the fever persisted. On day 10 of illness, he was hospitalized. Examination: temperature 39.8°C, heart rate 92 bpm, spleen palpable 1 cm below the costal margin (soft consistency), blood culture (negative). Typhoid fever is highly suspected clinically. Which test should be performed next for definitive diagnosis?