Liver Function Tests: ALT, AST, ALP, Bilirubin, and Albumin
The LFT panel mixes leakage enzymes with true function markers. Here is what separates the two, and why the ALT/AST pattern matters more than either number alone.
The panel labelled "liver function tests" is slightly misnamed. Most of what it contains does not measure function at all — it measures leakage. Separating the two groups is the key to reading an LFT report.
The enzymes: markers of leakage
ALT and AST are enzymes that live inside liver cells. They appear in blood when those cells are injured and their contents escape, so a raised value indicates that damage is occurring — not how well the liver is doing its job. A severely scarred liver with few remaining cells to injure can show near-normal enzymes.
- ALT (SGPT) is found predominantly in the liver, making it the more liver-specific of the two
- AST (SGOT) is also present in heart, skeletal muscle, and red blood cells, so it can rise from causes that have nothing to do with the liver — including strenuous exercise or a haemolysed sample
Because their distribution differs, the ratio between them is often more informative than either number. Clinicians read the pattern of which enzyme predominates alongside the clinical picture, rather than treating the two as one measurement.
ALP and GGT: the drainage pattern
Alkaline phosphatase concentrates around the bile ducts, so it rises when bile flow is obstructed rather than when liver cells are injured. It has an important confounder: ALP is also produced by bone, and is normally elevated during growth in children and adolescents and in pregnancy.
GGT is typically ordered to resolve that ambiguity. It rises with bile duct problems but not with bone turnover, so a raised ALP with a normal GGT points away from the liver.
Bilirubin
Bilirubin is the pigment left over when red blood cells are broken down, and the liver processes it for excretion in bile. Reports usually split it:
| Fraction | Stage | Rises when |
|---|---|---|
| Unconjugated (indirect) | Before liver processing | Red cell breakdown outpaces processing, or uptake is reduced |
| Conjugated (direct) | After liver processing | Processed bilirubin cannot drain away normally |
| Total | Both combined | Either mechanism |
Which fraction dominates is what makes the split worth reporting — a raised total tells you far less than knowing where in the pathway it accumulated.
The two that really do measure function
Albumin is the main protein the liver manufactures, and prothrombin time / INR reflects clotting factors it also produces. Because both are products the liver has to synthesise, they measure genuine synthetic capacity rather than injury.
They differ in speed. Albumin has a half-life of around twenty days, so it responds slowly and reflects longer-term capacity. Clotting factors turn over in hours to days, so INR moves quickly and is the more responsive marker of acute change. Albumin also falls for reasons unrelated to the liver — malnutrition, kidney loss, and inflammation among them.
Reading the panel as a pattern
No single line on an LFT is interpretable alone. What clinicians read is the shape: which group is raised, by how much relative to the others, whether the synthetic markers have moved, and how all of that compares with previous panels.
Reference ranges for these markers
Open the library entry for the published range, sample type, and citations.
Keep your own results in one place
File the reports behind these numbers under each family member and watch repeat values build into a trend. Free to start.
Educational content only. This article describes what a test measures and how published reference ranges are defined. It is not a diagnosis, does not interpret your personal results, and is not a substitute for a qualified doctor. Reference thresholds cited are those published by bodies such as the WHO, NIH, and CDC, and can differ between laboratories.
