A 54-year-old woman presented with fatigue and dark urine for 8 weeks. On physical examination, she has positive scleral icterus. Initial investigations showed: Alanine aminotransferase 78 U/L (10 – 40) Aspartate aminotransferase 88 U/L (10 – 30) Serum bilirubin 20.8 mg /dL (0.3 – 1.2) Direct bilirubin 12.0 mg/dL (0 – 0.2) Alkaline phosphatase 145 U/L (25 – 100) Serum albumin 3.1 g/dL (3.4 – 4.8) Serum protein 8.7 g/dL (6.4 – 8.3) Prothrombin time 18 sec (18 – 22 ) Which of the following tests should be performed next ?