any laboratory procedure that measures and assesses various aspects of liver function.

Because of the diversity of liver function and the varied and complicated metabolic processes that may be affected by disease states, more than 100 tests have been devised to test liver function. These reactions bear upon the metabolism of proteins, fats, carbohydrates, bile, and the detoxification and clearance of drugs and toxic chemicals performed by the liver. Since most of the substances metabolized by the liver enter the systemic circulation, the selective determination of the concentration of some of them in the blood usually gives valuable diagnostic information about a patient's liver. Urinary values are sometimes also obtained to corroborate blood-serum findings. In clinical practice, some of the more important blood-serum substances are ammonia, urea, amino acids, proteins, free and esterified cholesterol, bilirubin, and various enzymes (notably, cholinesterase, ceruloplasmin, the transaminases, and alkaline phosphatase). In assessing liver metabolism of sugars, tolerance tests are helpful (see glucose tolerance test).

Tests measuring the capacity of the liver to detoxify and clear toxic compounds involve the selective use of such test substances as hippuric acid and Bromsulphalein. Other diagnostic measures of liver function are based on the following: X-ray, following the opacification of liver structures with a radiopaque substance; biopsy; the administration of a radioactive compound that is absorbed to different degrees by healthy and diseased liver cells; and the mapping of the differential distribution of radioactivity. Techniques such as CAT (computerized axial tomography), MRI (magnetic resonance imaging), ultrasound, and nuclear radioisotope scanning have greatly assisted in determining the size of the liver, the presence or absence of cirrhosis, and the presence or absence of benign or malignant neoplasms.