MRCP2-1949

A 35-year-old woman is brought to the Emergency Department after a paracetamol overdose. She admits to taking 40 tablets of 500 mg each with the intention of ending her own life. This occurred 4 hours ago and she has been experiencing increasing abdominal pain and vomiting. On examination, she appears distressed and her right upper quadrant is tender. She has no signs of jaundice and her GCS is 15/15.

Which test is the most reliable indicator of the severity of liver damage in this patient?