AST

AST is an enzyme from the metabolism of amino acids, and it sits wherever a lot of energy is converted: in your liver cells, in your skeletal muscles, in your heart muscle, and to a lesser extent in your kidneys and red blood cells. It leaks into your blood as soon as cells in one of those tissues are damaged. So the value says cells are under strain somewhere, and unlike ALT it does not immediately say where. That is why AST and ALT are always read together. ALT sits almost only in the liver; AST sits in the liver and in the muscles. If AST rises while ALT stays normal, it comes from the muscles, and in anyone who does sport that is the most common situation: a hard strength session or a long run lets AST double or triple for a few days without anything being wrong with the liver. If both rise, it is the liver, and if AST rises more than ALT with a high gamma-GT alongside, alcohol is the first suspect. In fatty liver it is the other way round: ALT sits higher than AST. Your report shows AST in U/L. Normal is up to about 35 U/L for a man from age 19 and up to 31 U/L for a woman; in men the limit sits higher because they have more muscle mass. AST disappears from the blood faster than ALT, in about a day, so a rise from training is gone after a few rest days.

For anyone who does sport, AST is the liver value that most often gives an unnecessary scare. After a hard session, a race or a new exercise the muscles leak enzymes, and AST then sits above the band for a few days, together with CK and LDH, while ALT and gamma-GT stay quiet. That is not liver but muscle recovery. Anyone who knows that has blood drawn on a rest day and reads the value in the right company; anyone who does not gets a liver work-up that leads nowhere. The second reason is alcohol. Alcohol damages the liver cells in a way that mainly frees AST, and it also depletes the vitamin B6 with which ALT is measured. So with regular drinking AST rises more than ALT, and a ratio of AST to ALT above 2 in a liver problem is the classic picture of alcohol. Together with a high gamma-GT and a high MCV that is a pattern that answers the question about drinking before you ask it, and it falls in the weeks after you cut down. The third reason is the same as with ALT: fatty liver, the most common liver condition, in which AST usually rises somewhat less than ALT. And because AST also sits in the heart muscle, it rises with damage to the heart, though that is measured with troponin nowadays and no longer with AST.

A high AST means cells under strain in liver, muscles or heart. In order of how often it occurs: hard training or muscle damage, fatty liver, alcohol, medicines or supplements, and rarely damage to the heart muscle or a viral hepatitis. The key is ALT beside it. High AST with normal ALT and normal gamma-GT is muscle; high AST and high ALT is liver; high AST that rises more than ALT, with high gamma-GT, is alcohol; high ALT that rises more than AST is fatty liver. A low AST means nothing. With little muscle mass or a vitamin B6 deficiency the value sits somewhat lower, and that is not a finding. Always read AST beside ALT and gamma-GT, and in an athlete beside CK, which shows muscle damage directly. Have blood drawn on a rest day, at least two days after hard exertion, and compare over time under the same conditions.