Step by Step
1
ALT and AST — hepatocyte damage
ALT (7-56 U/L) is the most specific marker for liver damage, rising with hepatocyte injury from hepatitis, cirrhosis, fatty liver, or drug toxicity. AST (10-40 U/L) is less specific, since it's also found in heart and muscle tissue. An AST:ALT ratio greater than 2:1 specifically suggests alcoholic hepatitis.
2
ALP and GGT — cholestasis
ALP (alkaline phosphatase, 44-147 U/L) rises with cholestasis (bile duct obstruction), bone disease, or pregnancy. GGT helps confirm that an elevated ALP is coming from a biliary (liver) source rather than bone.
3
Bilirubin — hemolysis versus obstruction
Total bilirubin (0.1-1.2 mg/dL): elevated unconjugated (indirect) bilirubin suggests hemolysis or impaired liver uptake; elevated conjugated (direct) bilirubin suggests hepatic or post-hepatic obstruction. Jaundice becomes visible once bilirubin exceeds about 2.5 mg/dL.
4
Albumin and PT/INR — synthetic function
Albumin (3.5-5.0 g/dL) reflects the liver's synthetic function — decreased albumin suggests liver failure or malnutrition. PT/INR reflects clotting factor synthesis — an elevated INR suggests liver failure or warfarin use.
Applied Walkthrough
1
A patient with heavy alcohol use shows elevated AST and ALT, with an AST:ALT ratio above 2:1 — a pattern specifically suggestive of alcoholic hepatitis rather than another cause of hepatocyte injury.
2
A different patient presents with jaundice and an elevated ALP; a follow-up GGT confirms the ALP elevation is coming from the liver (biliary source) rather than bone — pointing toward cholestasis, likely from bile duct obstruction.
3
This same patient's bilirubin workup shows elevated conjugated (direct) bilirubin specifically, further supporting an obstructive process (rather than hemolysis, which would elevate unconjugated bilirubin instead).
4
A third patient with advanced cirrhosis shows low albumin and an elevated INR — both reflecting impaired synthetic liver function, since the liver can no longer produce adequate albumin or clotting factors, distinct from the injury and cholestasis markers seen in the other two patients.
Exam Application
Exams test whether you can distinguish hepatocellular injury (ALT, AST) from cholestasis (ALP, GGT) from synthetic function (albumin, PT/INR), and whether you can use the AST:ALT ratio and bilirubin fractionation to narrow down a specific cause.
⚠ Common Trap
The most common trap is treating all LFT abnormalities as reflecting the same underlying process — elevated ALT/AST reflects active hepatocyte damage, elevated ALP/GGT reflects bile flow obstruction, and abnormal albumin/PT-INR reflects impaired synthetic function; these are three distinct categories of liver dysfunction.
✓ Quick Self-Check
1. Which liver enzyme is most specific for hepatocyte damage?
ALT.
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2. What does an AST:ALT ratio greater than 2:1 suggest?
Alcoholic hepatitis.
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3. What does elevated ALP suggest, and what test confirms a liver (rather than bone) source?
Cholestasis (bile duct obstruction); GGT confirms a biliary source.
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4. What does elevated conjugated (direct) bilirubin suggest, versus unconjugated (indirect)?
Conjugated suggests hepatic or post-hepatic obstruction; unconjugated suggests hemolysis or impaired liver uptake.
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5. What do albumin and PT/INR reflect about liver function?
The liver's synthetic function — low albumin or elevated INR suggests liver failure (or malnutrition/warfarin, respectively).
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