Enzymes are not a function test
ALT, AST, GGT and alkaline phosphatase are enzymes that live inside liver cells or in the bile ducts. When those cells are irritated, some of the enzyme leaks into the blood and the number rises. That is an irritation signal, not a measure of how well the organ is doing its job.
The lines that do describe function are further down: albumin, which the liver makes, and the clotting figures, which depend on proteins the liver makes. A panel with high enzymes and normal albumin says something quite different from one with normal enzymes and low albumin.
This distinction explains a common surprise. Someone can have a mildly raised ALT with a liver that is working perfectly well, and the report is not contradicting itself.
Why GGT is the most misread line
GGT has a reputation as an alcohol test. It is not one. A review of alcohol biomarkers is explicit that liver enzyme measurements lack specificity for ethanol, which is why several other tests are needed to sort out competing causes.
Excess weight is an increasingly common cause of raised liver enzymes on its own, and the same review notes that ALT and GGT activity also tracks insulin resistance and metabolic syndrome, and even predicts outcomes unrelated to the liver. A study looking specifically for a drinking marker found GGT influenced by several factors at once, while a specialised marker, carbohydrate-deficient transferrin, tracked consumption independently of liver damage.
So a raised GGT means something is making your liver work. Alcohol is one candidate; weight, several medicines, bile duct problems and fatty liver are others.
How the lines are read together
ALT and AST are read as a pair, and their ratio carries information that neither carries alone. GGT is read next to alkaline phosphatase: when both rise together, attention moves towards the bile ducts rather than the liver cells themselves.
Bilirubin is read with the rest of the panel and with a blood count, because it also rises when red cells break down faster than usual, which has nothing to do with the liver being damaged.
And every one of these lines is read against what you were doing: a heavy meal, intense exercise, a recent viral infection and several common medicines all move them.
What to do with a mildly raised enzyme
Mild elevations are common and often transient. The usual next step is not a scan but a repeat after a few weeks, with the obvious contributors set aside in the meantime.
What matters is the pattern and the trend: which enzymes rose, in what proportion to each other, and whether the same picture is still there on the second sample.