Hepatology
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What Do Liver Tests Mean?

by Anne Larson, MD

"Your transaminases are up a little bit, but fortunately the pro time is normal as well as your alk phos, so there is no evidence of cholestasis." Good news or bad news? Even Webster's Unabridged Dictionary would not help a patient to understand the jargon. Yet some doctors speak this language to patients with liver disease. There are two solutions: get doctors to change (lots of luck) or learn the vocabulary (more likely solution). Several terms for standard liver tests are used often in conversation with patients with liver disease: transaminases, alkaline phosphatase (or "alk phos"), gamma-glutamyl transpeptidase (GGT), bilirubin ("bili"), albumin, and prothrombin time ("pro time" or "PT"). All are measured in the blood or serum (what's left of the blood after the cells are removed). The first three are enzymes which leak out of or are produced by injured liver cells and are, therefore, a sign of ongoing liver cell damage. The last three measure products excreted by or produced by the liver and hence reflect true liver "function".

Transaminases are of two types, abbreviated SGOT (or AST) or SGPT (or ALT) by clinical chemists, and they both indicate cell damage. These enzymes "leak" into the bloodstream from the damaged cells. Extremely high levels (often over 1000 units) occur in the blood in any condition characterized by acute damage of liver cells (as in acute viral hepatitis, exposure to chemical toxins, or severe allergic reactions to drugs). Lower levels occur with chronic ongoing damage (50-300 units). It is typically the test which is elevated in chronic active hepatitis from whatever cause.

Alkaline Phosphatase is an enzyme which comes from liver, bone, placenta, and intestine. When from the liver, high levels usually mean some difficulty with excretion of substances into the bile (rather than diffuse liver cell injury). Thus, it is increased in any condition which obstructs the flow of bile (for instance, a gallstone or tumor blocking the bile ducts). Also, certain disease and toxins or drugs may interfere with the excretory function of individual liver cells (so-called intrahepatic cholestasis) and cause increases of alkaline phosphatase. In this case, there is no damage to the machinery of the liver cell which makes important body proteins or other chemicals - only to the excretory machinery of the liver cells. Since bilirubin is one of the main body products excreted in bile, it is usually elevated in the blood in "cholestatic states." Cholestasis is the term used for a pattern of liver tests which most closely resembles selective damage to the excretory function of the liver (thinking of what you have read so far, this would be a pattern with small transaminase elevations and large elevations of alkaline phosphatase and bilirubin).

Gamma-Glutamyl transpeptidase (GGT) is a liver cell enzyme elevated both in liver cell damage and in cholestasis. It tends to be more sensitive than transaminases, but its major use is to distinguish the source of an elevated alkaline phosphatase (from liver, bone, placenta, or gut?) in difficult cases. Thus, if both the alkaline phosphatase and the GGT are elevated, then the alkaline phosphatase probably comes from the liver.

Two main functions of the liver are excretion of waste products and manufacture of necessary body proteins. This proteins include bilirubin which is, therefore, a true "liver function" test (rather than an indicator of liver inflammation or damage). Both albumin and prothrombin are proteins made only by the liver. Blood levels tend to be low in liver disease because the damaged liver cannot produce them. They are also indicators of true "liver function". The albumin is measured chemically. The prothrombin (a necessary factor for blood coagulation) is measured by a standardized test of clotting (prothrombin time). Prothrombin is doubly important in severe liver damage because inadequate synthesis of clotting factors leads to excessive bleeding - one of the major causes of death from liver failure.

So now you may have some understanding of what the doctor at the beginning of the article meant. He/she told said that you have inflammation (bad news) which is not yet compromising vital functions (good news). Also, he/she is sure the disease is not interfering with the excretory function of the liver (good news). These tests can be very useful in helping your doctor determine the cause of liver disease and the pattern of liver damage.

Adapted from W.G.M. Hardison, M.D.
Professor of Medicine
University of California, San Diego

 
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