What is ALT?

ALT is held more exclusively in the liver than the other enzymes on a routine panel, which is why it is the one most used to ask whether the liver is under strain. The amount in blood tracks how fast liver cells are being injured and replaced, not any one disease.

AST leaks from injured cells in much the same way, but it is also stored in muscle and red blood cells, so reading the two together narrows down the source. ALP and Bilirubin instead follow bile as it drains out of the liver, so they move when the drainage is obstructed rather than when liver cells are injured.

Alcohol injures liver cells directly, and so do viral hepatitis and other infections; medicines and supplements can injure them too. Diet acts more slowly, through fat accumulating in the liver. Sex and body composition set the level ALT sits at when nothing is wrong, so a change matters against a person's own earlier readings.

Liver cell injury pushes ALT up within a day or two, and one to a few weeks pass before it settles as damaged cells are cleared. A single measurement can therefore catch ALT still falling from an injury weeks earlier, reading as raised while the liver is already recovering. Testing about every four months separates a one-off event from a pattern holding across months. This is Do Health's monitoring schedule, not a general recommendation for everyone.