A 29-year-old male presents with 3 days of fever, headache, fatigue, decreased appetite, calf pain while walking, and cough. Physical examination: temperature 39.8°C, flushed face, conjunctival congestion, normal vision, bilateral inguinal lymphadenopathy, marked tenderness of the gastrocnemius muscle, no meningeal signs or pathological reflexes. Liver palpable 1.5 cm below the costal margin. White blood cell (WBC) count 12.7×10<sup>9</sup>/L (neutrophils 74%, lymphocytes 21%, eosinophils 3%, monocytes 2%). The patient frequently worked in rice paddies prior to symptom onset. Which laboratory test is most meaningful for diagnosis?