A 31-year-old previously healthy primagravida presents in the 29th week of gestation with progressively worsening pruritus (scored by the patient using a Visual Analog Scale 6/10). Screening blood tests prior to pregnancy and during the early pregnancy were all within the normal range and the previous medical history is devoid of any liver disease. Screening blood tests reveal the following abnormal results: ALT 2.1 X ULN; AST 1.6 x ULN; bilirubin 1.2 x ULN. Other routine tests are within normal limits. Liver ultrasound is unremarkable and shows no signs of liver disease.

The result of which blood test will help to confirm the suspected diagnosis?