A single elevated liver enzyme tells you something is wrong; the PATTERN across several values tells you what specifically is wrong.
ALT
Liver-spec.
AST
Liver+muscle
ALP
Bile/bone
Bili
Jaundice
Alb
Synth.
📖 Full Breakdown
Five values, and how their relative pattern points to a specific diagnosis
No single LFT value works in isolation — the ratio and combination of values is where the diagnostic power lies.
ALT
Most specific for liver damage
Normal 7-56 U/L. Because ALT is found predominantly in liver cells, an elevated ALT points more specifically to liver injury than AST does.
AST
Liver AND heart/muscle
Normal 10-40 U/L. Since AST is also present in heart and muscle tissue, an elevated AST alone is less liver-specific — but the AST:ALT RATIO becomes diagnostically useful: a ratio greater than 2:1 suggests alcoholic hepatitis specifically.
ALP (Alkaline Phosphatase)
Liver AND bone
Elevated in cholestasis (bile flow obstruction) as well as bone diseases like Paget's disease — context from other labs is needed to know which source is responsible.
Bilirubin
Jaundice marker
Direct (conjugated) bilirubin elevation points to liver or post-hepatic causes; indirect (unconjugated) elevation points to pre-hepatic causes like hemolysis — the same lab test can indicate opposite problem locations depending on which fraction is elevated.
Albumin and PT/INR
Synthetic function
Low albumin and elevated PT/INR both reflect the liver's reduced ability to synthesize proteins — these measure the liver's FUNCTION, not just cell damage, distinguishing them fundamentally from ALT/AST.
🩺 Clinical / Exam Application
A patient with a long history of heavy alcohol use has an AST of 150 and an ALT of 60 — an AST:ALT ratio over 2:1. Because this specific ratio pattern (rather than either value alone) is classically associated with alcoholic hepatitis, this single calculation from two routine numbers points toward a specific cause without needing additional specialized tests. Isolated elevated ALT or AST alone wouldn't tell this story — it's the ratio between them that carries the diagnostic weight.
⚠️ Exam Alert
A frequently tested distinction: ALT and AST measure liver cell DAMAGE (enzyme leakage from injured cells), while albumin and PT/INR measure liver FUNCTION (the liver's ability to synthesize proteins) — a patient can have normal ALT/AST but still have poor synthetic function in chronic, burned-out liver disease.
🚧 Common Trap
Don't assume ALP elevation always means liver disease. Because ALP is also produced by bone, an isolated ALP elevation requires additional tests (like GGT, which is liver-specific) to determine whether the source is hepatic or skeletal.
✅ Quick Check
A patient has elevated ALP but normal ALT and AST. What additional consideration would help determine whether this reflects liver or bone pathology?
📝 Exam Prep
Common Exam Questions
❓ What is the role of the liver in digestion?
✅ The liver produces bile (stored in the gallbladder), metabolizes nutrients absorbed from the gut, detoxifies drugs and toxins, produces clotting factors, and metabolizes bilirubin from old red blood cells.
❓ What does an AST:ALT ratio greater than 2:1 suggest?
✅ It classically suggests alcoholic hepatitis, since alcohol-related liver damage tends to elevate AST more than ALT, unlike most other causes of liver injury where ALT rises more.