This is a multicenter, open label, non-randomized study of the ALF-MBT to assess functional trends of liver metabolism in patients diagnosed with ALF. Up to 200 evaluable male and female patients with acute liver failure present at the time of enrollment into the ALFSG Registry will be consecutively enrolled. An evaluable patient is one who has completed one or more ALF-MBTs measured for a minimum of 30 (and ideally 60) minutes after administration of 13C-Methacetin. Study sites will include up to 12 of the clinical sites located in the United States that are involved in the Acute Liver Failure Study Group.