Interpreting LFTs Flashcards
What are the 2 reasons why LFTs are requested?
- to confirm suspicion of potential liver disease
- to distinguish between hepatocellular injury (hepatic jaundice) and cholestasis (post-hepatic / obstructive jaundice)
What are the 7 parameters measured in an LFT and what do they indicate?
- alanine transaminase (ALT)
- aspartate aminotransferase (AST)
- alkaline phosphatase (ALP)
- gamma-glutamyltransferase (GGT)
- albumin
- bilirubin
- prothrombin time
- ALT, AST, ALP & GGT are used to distinguish between cholestasis & hepatocellular injury
- bilirubin, albumin & PT measure the liver’s synthetic function
What is the first stage in interpreting LFTs?
- look at ALP and ALT
- decide whether there is more than a 10-fold rise in ALT
- and/or more than a 3-fold rise in ALP
How can ALP and ALT be compared to determine the cause of jaundice?
ALT:
- a marker of hepatocellular injury
- a > 10-fold increase in ALT + < 3-fold increase in ALP = hepatocellular injury
ALP:
- a marker of cholestasis
- a > 3-fold increase in ALP + < 10-fold increase in ALT = cholestasis
it is possible to have a mixed picture with hepatocellular injury & cholestasis
After assessing ALT & ALP, which marker should be reviewed?
GGT
- important to review when there is a raised ALP
- GGT can be raised in response to alcohol or drugs (e.g. phenytoin)
!! a markedly raised ALP + raised GGT is highly suggestive of cholestasis !!
What does a rise in ALP without a rise in GGT indicate?
- ALP is present in bone, so anything that causes bone breakdown can raise ALP
- bony metastases / primary bone tumours
- vitamin D deficiency
- recent fractures
- renal osteodystrophy
What does it suggest if a patient is jaundiced but ALT and ALP levels are normal?
- an isolated rise in bilirubin suggests a pre-hepatic cause of jaundice
- e.g. Gilbert’s syndrome
- or haemolysis
What are the liver’s main synthetic functions and the blood tests that can be used to assess them?
conjugation / elimination of bilirubin:
- assessed via serum bilirubin
synthesis of albumin:
- assessed via serum albumin
synthesis of clotting factors:
- assessed via prothrombin time (PT)
gluconeogenesis:
- assessed via serum blood glucose
Which symptom can be used to distinguish between conjugated and unconjugated hyperbilirubinaemia?
colour of the urine:
- unconjugated bilirubin is NOT water soluble and does NOT affect the colour of the urine
- conjugated bilirubin passes into the urine as urobilinogen and causes the urine to become darker
unconjugated bilirubin is taken up by the liver and conjugated
How can the colour of the stools be used to differentiate causes of jaundice?
- fat cannot be absorbed when bile / pancreatic lipases cannot reach the bowel
- this occurs in obstructive post-hepatic pathology
- the stools appear pale, bulky and difficult to flush
How can the colour of the urine + stools be used to give an indication of the cause of jaundice?
pre-hepatic jaundice:
- normal urine + normal stools
hepatic jaundice:
- dark urine + normal stools
post-hepatic jaundice:
- dark urine + pale stools
dark urine = conjugated hyperbilirubinaemia
What are the causes of unconjugated hyperbilirubinaemia?
- haemolysis (haemolytic anaemia)
- impaired hepatic uptake (e.g. drugs, congestive HF)
- impaired conjugation (e.g. Gilbert’s syndrome)
What are the causes of conjugated hyperbilirubinaemia?
- hepatocellular injury
- cholestasis
Why might there be a fall in albumin levels?
- liver disease resulting in decreased production of albumin (e.g. cirrhosis)
- inflammation triggering an acute phase response which temporarily decreases production of albumin
- protein-losing enteropathies / nephrotic syndrome leading to excessive loss of albumin
albumin is important for maintaining the oncotic pressure of blood
What does prothrombin time measure?
- a measure of the blood’s coagulation tendency
- it specifically looks at the extrinsic pathway
How can LFTs be used to distinguish between acute and chronic hepatocellular damage?
