A "liver function test" mostly measures enzymes. MedlinePlus groups them together: ALT, AST, ALP and GGT "are enzymes that are mainly made in your liver."
The panel measures the concentration of those enzymes in your blood — which is a different thing from measuring how well the liver is working, despite the name on the report.
The four rows
ALT — alanine transaminase.
AST — aspartate aminotransferase.
ALP — alkaline phosphatase.
GGT — gamma-glutamyl transferase.
MedlinePlus describes all four with the same phrase: enzymes "mainly made in your liver". The word doing the work there is mainly — these enzymes are not exclusive to one organ, which is why a panel reports several rather than one.
The naming problem
This panel is where lab naming is at its worst, and it matters if you are comparing reports.
ALT is also printed as SGPT — serum glutamic-pyruvic transaminase, an older name for the
same enzyme. AST is also printed as SGOT. Many reports print both forms together, as
SGPT (ALT) or SGOT/AST.
They are not two tests. A report using SGPT and a report using ALT are measuring the same thing, and a history assembled across labs will contain both spellings for one marker.
LDH — lactate dehydrogenase — sometimes appears on the same panel. It is a separate enzyme, found widely through the body rather than concentrated in one organ.
Why several enzymes rather than one
Because they are not distributed identically. Enzymes concentrated in different tissues, released in different circumstances, produce different patterns across the four rows — and the pattern carries more information than any single row.
Reading that pattern is clinical work. This guide is about what the columns are, not what a combination of them indicates.
Reading them across reports
Two practical things.
Check the name mapping first: an SGPT row and an ALT row belong on the same trend line, and a
tool that treats them as separate markers will show two half-length histories instead of one.
And check the units and interval, which vary between laboratories for these enzymes as much as for anything else — the interval that applies is the one printed on the report the value came from.