Measurement in a specified sample of liver of the amount of ezymatic activity of ornithine decarboxylase, that is, the ability of the protein to catalyze the decarboxylation of ornithine (a product of the urea cycle) to form putrescine, the rate-limiting and first committed step in polyamine synthesis. The liver is the large, dark-red organ located in the cranial portion of the abdomen of vertebrates, just behind or below the diaphragm, the functions of which include storage and filtration of blood; secretion of bile; detoxification of noxious substances; conversion of sugars into glycogen; synthesis and breakdown of fats and temporary storage of fatty acids; and synthesis of serum proteins.