Liver Function Tests (LFTs) Flashcards
Why investigate LFTs?
LFTs are requested for two primary reasons:
- To confirm a clinical suspicion of potential liver injury or disease
- To distinguish between hepatocellular injury (hepatic jaundice) and cholestasis (post-hepatic or obstructive jaundice)
What blood tests are used to assess liver function?
- Alanine transaminase (ALT)
- Aspartate aminotransferase (AST)
- Alkaline phosphatase (ALP)
- Gamma-glutamyltransferase (GGT)
- Bilirubin
- Albumin
- Prothrombin time (PT)
Which LFTs are used to distinguish between hepatocellular damage and cholestasis?
ALT, AST, ALP and GGT
Which LFTs are used to assess the liver’s synthetic function?
Bilirubin, albumin and PT
Briefly describe ALT
ALT is found in high concentrations within hepatocytes and enters the blood following hepatocellular injury. It is, therefore, a useful marker of hepatocellular injury.
Briefly describe ALP
ALP is particularly concentrated in the liver, bile duct and bone tissues. ALP is often raised in liver pathology due to increased synthesis in response to cholestasis. As a result, ALP is a useful indirect marker of cholestasis.
How does ALT and ALP show in hepatocellular injury?
Greater than 10-fold increase in ALT and a less than 3-fold increase in ALP suggests a predominantly hepatocellular injury.
ALT>ALP
How does ALT and ALP show in cholestasis?
Less than 10-fold increase in ALT and a more than 3-fold increase in ALP suggests cholestasis.
ALT
Briefly describe gamma-glutamyl transferase
A raised GGT can be suggestive of biliary epithelial damage and bile flow obstruction. It can also be raised in response to alcohol and drugs such as phenytoin. A markedly raised ALP with a raised GGT is highly suggestive of cholestasis.
What may an isolated rise in ALP show? And what may cause this?
A raised ALP in the absence of a raised GGT should raise your suspicion of non-hepatobiliary pathology. Alkaline phosphatase is also present in bone and therefore anything that leads to increased bone breakdown can elevate ALP.
Causes:
- Bony metastases or primary bone tumours (e.g. sarcoma)
- Vitamin D deficiency
- Recent bone fractures
- Renal osteodystrophy
What if the patient is jaundiced but ALT and ALP levels are normal?
An isolated rise in bilirubin is suggestive of a pre-hepatic cause of jaundice.
Causes of an isolated rise in bilirubin include:
- Gilbert’s syndrome: the most common cause
- Haemolysis: check a blood film, full blood count, reticulocyte count, haptoglobin and LDH levels to confirm
What are the synthetic functions of the liver?
The liver’s main synthetic functions include:
- Conjugation and elimination of bilirubin
- Synthesis of albumin
- Synthesis of clotting factors
- Gluconeogenesis
What is bilirubin? And what is hyperbilirubinaemia?
Bilirubin is a breakdown product of haemoglobin. Unconjugated bilirubin is taken up by the liver and then conjugated. Hyperbilirubinaemia may not always cause clinically-apparent jaundice (usually visible >60 umol/l).
How can the colour of urine be used to differentiate between conjugated and unconjugated bilirubin?
Unconjugated bilirubin is water-insoluble and, therefore, doesn’t affect the colour of the patient’s urine.
Conjugated bilirubin, however, can pass into the urine as urobilinogen, causing the urine to become darker.
How can the colour of urine and stools indicate the cause of jaundice?
Normal urine + normal stools = pre-hepatic cause
Dark urine + normal stools = hepatic cause
Dark urine + pale stools = post-hepatic cause (obstructive)
How does acute hepatocellular damage appear with regards to ALT, ALP, GGT and bilirubin?
How does chronic hepatocellular damage appear with regards to ALT, ALP, GGT and bilirubin?
How does cholestasis appear with regards to ALT, ALP, GGT and bilirubin?
