A 42-year-old male presents to the emergency department with 2 days of vomiting and diarrhea, and half a day of confusion and restlessness. Examination: BP 110/70 mmHg, clouding of consciousness, moderate scleral icterus, multiple spider angiomas on the neck, no abnormalities in heart and lungs, soft abdomen, liver not palpable below the costal margin, spleen 3 cm below the costal margin, ecchymosis at the puncture site of the upper limb. Hb 90g/L, WBC 3.2×10⁹/L, blood glucose 7.0 mmol/L, glycosuria (+), ketonuria (-), urine microscopy (-). The most valuable auxiliary test for diagnosing hepatic encephalopathy is: