DEAR DR. DONOHUE: Since starting Zocor to lower my cholesterol, my doctor has had me get liver tests. At first I was getting a test called AST. Now I am getting one called ALT. Why was the change made? Is the second one better than the first? Is there a difference in price? – M.F.

ANSWER: AST is aspartate aminotransferase, an enzyme found in liver, muscle, kidney and red blood cells. ALT is alanine aminotransferase, another enzyme. It is found mostly in liver cells. Enzymes are proteins inside cells that speed up the cells’ chemistry. When cells are damaged, their enzymes leak into the blood and can be detected by the lab.

Since ALT is confined mostly to the liver, a rise in its blood level points to the liver as the source of cell damage. AST is found in other sites, and trouble at any one of those sites can cause a rise. ALT, therefore, is a more specific test for liver damage. In actual practice, both tests are usually ordered together. They back each other up. The cost of checking both isn’t more expensive than is checking for only one. I don’t know why your doctor orders only one.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com