TESTS OF HEPATIC FUNCTION



Although the liver performs a great variety of presumably testable functions, it has proved dif­ficult to devise a test that is simple, cheap, re­producible, and noninvasive and that accurately reflects hepatic capacity for all functions. Instead,currently available tests of liver function are in­direct, static measurements of serum levels of compounds that are synthesized, metabolized, and/or excreted by the liver. In interpreting these tests, it is important to remember that the liver has a large reserve capacity, and therefore “func­tion” tests may remain relatively normal until liver dysfunction is severe. Table 43-2 outlines the most available and useful liver function tests. The serum albumin level and prothrombin time both reflect the hepatic capacity for protein syn­thesis, although changes in these proteins are not specific for liver disease. The prothrombin time responds rapidly to altered hepatic function be­cause the serum half-lives of Factors II and VII are short (hours). In contrast, the serum half-life of albumin is 14 to 20 days, and serum levels fall only with prolonged, severe liver dysfunction.

Serum bile acid levels, particularly when meas­ured two hours after a meal, have proved to be the most sensitive test of liver disease, and this is due to the high efficiency with which the liver nor­mally extracts bile acids from portal blood. Small changes in hepatic blood flow, portosystemic shunting, or liver function all result in a substan­tial elevation of serum bile acid levels, while ter­minal ileal dysfunction (e.g., Crohn’s disease] leads to fecal loss of bile acids and decreased serum levels. Although exquisitely sensitive, bile acid levels are nonspecific and fail to reflect ac­curately overall liver function.

The 14C-aminopyrine breath test was originally developed as a true test of liver function. It meas­ures the rate at which the liver metabolizes 14C-labeled aminopyrine to 14C02, which is collected and measured in exhaled breath. This test is per­formed in some academic centers and may be use­ful in following the progression of liver disease in an individual patient.