LFT interpretation

Medical Students’ Guide to LFT interpretation

Many patients in hospital have liver function tests (LFTs) ordered for a variety of reasons. It is important to note that abnormal LFTs are often but not always indicative of liver disease; also, normal LFTs do not always mean that the liver is normal.


Bilirubin 3-17 µmol/L
Alanine Aminotransferase (ALT) 3-35 iu/L
Aspartate Aminotransferase (AST) 3-35 iu/L
Alkaline Phosphatase (ALP) 30-300 iu/L
Gamma-glutamyl Transpeptidase (γGT) 11-51 iu/L
Albumin 35-50 g/L
Prothrombin Time (PT) 10-14 seconds

[N.B. normal results are adjusted for each laboratory]



Bilirubin
- produced from breakdown of haem in red blood cells
- once levels are increased to above 35 µmol/L, jaundice can be seen
- causes of a raised plasma bilirubin can be classified as
1. Pre-hepatic - haemolysis
2. Hepatic - viruses (e.g. hepatitis), drugs (e.g. statins), cirrhosis
3. Post-hepatic - gallstones in common bile duct

ALT
- enzyme located in cytosol of liver cells
- levels increased by any condition that damages the liver cells and results in the enzyme leaking out
- the ALT level may or may not correlate with the degree of cell death or inflammation.
- ALT is the most sensitive marker for liver cell damage.

AST
- enzyme located in cytosol and mitochondria of liver cells
- less sensitive for liver damage than ALT
- levels elevated in liver disease, myocardial infarction, skeletal muscle damage
- beware as in chronic liver disease, levels of AST/ALT are less sensitive

ALP
- enzyme associated with biliary tree, but is not specific
- elevated in liver disease (cholestasis), bone disease, pregnancy (placenta), renal damage

γGT
- enzyme associated with bile ducts but is not very specific
- elevated in liver disease (cholestasis)
- an elevation with no rise in ALP indicates alcohol-induced enzyme induction

Albumin
- albumin is synthesized in the liver
- low albumin: liver disease, nephrotic syndrome, malabsorption, malnutrition
- high albumin: dehydration

Prothrombin Time
- the liver synthesizes blood clotting factors II, V, VII, IX and X
- PTT is a test that measures the clotting time of plasma
- not specific for liver disease
- increased in liver disease, vitamin K deficiency

Summary

Markers of hepatocellular damage: AST and ALT
Markers of cholestasis: ALP and GGT
Markers of liver function: bilirubin, albumin, prothrombin time