Clinical Biochemistry: Laboratory Investigation of Liver & GI Tract Disease Flashcards
Describe the structure of the liver
- The liver is the largest organ in the body
- Located in right upper quadrant of abdomen
- Comprised of large right lobe and smaller left lobe
- Lobes consist of lobules - sheets of hepatocytes
- Has dual blood supply – 2/3 comes from the gut via the hepatic portal vein (nutrient rich) and 1/3 from the hepatic artery (oxygen rich)
- Blood leaves the liver through the hepatic veins
- Substances for excretion from the liver are secreted from hepatocytes into canaliculi.
- The bile canaliculi merge and form bile ductules, which subsequently merge to become a bile duct and eventually become the common hepatic duct.
What are the major functions of the liver?
- Carbohydrate metabolism
- Fat metabolism
- Protein metabolism
- Synthesis of plasma proteins
- Hormone metabolism
- Metabolism and excretion of drugs and foreign compounds
- Storage – glycogen, vitamin A and B12, plus iron and copper
- Metabolism and excretion of bilirubin
What are some of the most common types of liver disease?
-
Hepatitis
- Inflammation of liver leading to damage of hepatocytes
-
Cholestasis
- Blockage of canaliculi
- Can be Intra-heptatic or extra-hepatic (in bile ducts)
-
Cirrhosis
- Increased fibrosis - leads to scarring
- Liver shrinkage
- Decreased hepatocellular function
- Obstruction of bile flow
-
Tumours
- Can be primary cancer (e.g. Hepatocellular carcinoma)
- More frequently secondary cancer of: colon, stomach, bronchus (known as liver metastases)
How do you biochemically assess liver function?
- Use a Liver Function Test (LFT)
- LFT useually tests for:
- Bilirubin
- Albumin
- Alanine aminotransferase (ALT) or Aspartate aminotransferase (AST)
- Alkaline phosphatase
- Gamma glutamyltransferase
Why are liver function tests not used for diagnosis of liver disease?
- Because they are insensitive indicators of liver ‘function’ so a single result rarely provides a diagnosis on its own
- This means you have to look for patterns of results that may indicate liver disease
What are liver function tests used for?
- Screening for the presence of liver disease
- Assessing prognosis
- Measuring the efficacy of treatments for liver disease
- Differential diagnosis: predominantly hepatic or cholestatic
- Monitoring disease progression
- Assessing severity, especially in patients with cirrhosis
Give an example of a typical inflammatory pattern that is seen on a liver function test
- Bilirubin: Either normal or slightly increased
- ALT: Massively increased
- ALP: Either normal or slightly increased
- Albumin: Normal
Give an example of a typical cholestatic pattern that is seen on a liver function test
- Bilirubin: Variable increase
- ALT: Either normal or slightly increased
- ALP: Usually massively increased
- Albumin: Normal
What type of liver disease will cause a change in albumin levels?
- Tends to decrease in chronic liver disease (cirrhosis)
What is Bilirubin?
- Yellow-orange pigment derived from the breakdown of haem
What are the 2 different forms of Bilirubin?
- Conjugated (direct-reacting bilirubin)
- Unconjugated (indirect-reacting bilirubin) - Very hydrophobic and so has to be conjugated in liver to become more water-soluble to allow it to be excreted
What measurements of bilirubin can be directly measured in the lab?
- Total bilirubin (usually <21 mmol/L)
- Conjugated (direct) bilirubin (usually <10 mmol/L)
How can unconjugated bilirubin be measured using total and conjugated bilirubin levels?
- Unconjugated bilirubin = Total bilirubin - Conjugated (direct) bilirubin
What form of bilirubin binds to which other liver protein? Why is this?
- Unconjugated bilirubin binds tightly but reversibly to albumin
- This occurs because unconjugated bilirubin is very hydrophobic and so binding to albumin allows it to be transported via the bloodstream to the liver to be conjugated
Describe the process of bilirubin metabolism
- RBC’s transported to spleen where they get broken down by reticuloendothelial cells
- Iron that’s formed from break down is re-utilised but haem gets broken down into bilirubin (unconjugated)
- Bilirubin binds to albumin which allows it to travel to the liver
- In the liver bilirubin gets conjugated to glucuronide via the enzyme UDP-glucuronyl transferase
- This conjugated bilirubin is now water soluble and is taken to the small intestine via the bile duct
- In the small intestine bilirubin gets converted to urobilinogen
- Urobilinogen can enter the liver via the extrahepatic circulation or it can enter the systemic circulation where it’ll eventually be excreted by the kidneys
- Most urobilinogen goes on to enter large intestine where bacteria converts it to stercobilin.
What is Jaundice?
- The yellow discolouration of tissue due to bilirubin deposition - imbalance between production & excretion
- This imbalance results in increased serum/plasma concentration of bilirubin
How high must the serum/plasma concentration of bilirubin be before symptoms are evident?
- 2x the upper reference of normal, >50 μmol/L.
Why is it important to determine whether there has been an increase in conjugated or unconjugated bilirubin in Jaundice?
- Because it is key in determining what is happening to the liver and where it’s being damaged
- If there’s an increase in unconjugated bilirubin it means production is increased beyond capacity of liver conjugation
- If there’s an increase in conjugated bilirubin it means there’s an obstruction of bilirubin flow
What are some of the causes of Jaundice?
-
Pre-hepatic: Excessive production of bilirubin e.g. due to excessive haemolysis (breakdown of RBCs)
- Can also be caused by Haemolytic anaemia, Crigler-Najjar syndrome and Gilbert’s syndrome
-
Intra-hepatic: Dysfunction of hepatic cells - can lead to some cholestasis due to scarring and constriction of caniculi
- Can also be caused by, Viral hepatitis, Drugs and Cirrhosis
-
Extra-hepatic: Blockage of Caniculi causing Obstruction of bilirubin drainage
- Can also be caused by Common duct stone, Carcinoma and Pancreatitis
What is Neonatal Jaundice?
- Jaundice in babies due to the incomplete maturation of the immune system (immaturity of bilirubin conjugation enzymes)
- Neonatal jaundice is transient (resolves in the first 10 days).
Why are high levels of unconjugated bilirubin very dangerous for a newborn?
- Because due to its hydrophobicity unconjugated bilirubin can cross the blood-brain-barrier and cause kernicterus - brain dysfunction caused by excess bilirubin
- This brain dysfunction occurs because bilirubin is toxic to the neurons of the brain
How is neonatal jaundice treated?
- Phototherapy with UV light – converts unconjugated bilirubin to water soluble, non-toxic conjugated form
What is it called when neonatal jaundice results in an increase in conjugated bilirubin?
- Pathological jaundice
What is Gilbert’s Syndrome?
- Benign liver disorder characterized by mild, fluctuating increases in unconjugated bilirubin
- It is caused by a genetic defect in the promoter gene for UDP-glucuronyl transferase
- This results in a decreased ability of the liver to conjugate bilirubin
What are the main functions of Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST)?
- They catalyse the transfer of amino groups between amino acids (α-amino acid -> α-oxo acid)
Where are ALT and AST distributed within the body?
- ALT is predominantly localised to liver
- AST has wide tissue distribution: located in heart, skeletal muscle, kidney, brain, erythrocytes, lung & liver
Are both ALT and AST cytosolic (found with cytosol of a cell)?
- Yes both are cytosolic but AST is also present in mitochondria
What is ALT used to identify?
- Used to identify liver damage arising from hepatocyte inflammation or necrosis
How can ALT/AST levels be used to differentiate between severe liver damage and cholestasis?
- ALT/AST values >20x the upper limit of normal (ULN) may occur with severe liver damage
- ALT/AST values <5x ULN may occur in cholestasis due to secondary damage to hepatocytes
What liver diseases result in a modest elevtaion of ALT/AST (5 x ULN)?
- Fatty liver
- Chronic viral hepatitis
- Prolonged Cholestatic liver disease
- Cirrhosis - In compensated cirrhosis values may be normal
What liver diseases result in a high elevation of ALT/AST (10-20x ULN)?
- Acute viral hepatitis
- Hepatic necrosis induced by drugs or toxins
- Ischaemic hepatitis induced by circulatory shock
What is the function of Alkaline Phosphatase (ALP)?
- It removes phosphate groups from proteins and nucleic acids in alkaline pH environments
Where is ALP distributed within the body?
- ALP isoforms mainly produced in liver and bone but also placental and intestinal forms
What type of liver disease is ALP a good marker for?
- Cholestasis (Bile duct obstruction) because this causes increased ALP synthesis and thus an increase in measured activity
How can the source of an elevated ALP be determined?
- Can be determined by gel electrophoresis which is used to separate ALP isoenzymes into liver, bone, and intestinal fractions.
Compare the following features between NAFLD and alcoholic liver disease:
- Body weight
- Fasting plasma glucose or HbA1c
- Reported daily alcohol intake
- ALT
- AST
- AST:ALT ratio
- GGT
- Triglycerides
- HDL cholesterol
- Mean corpuscular volume
What ELF score indicates high risk of advanced fibrosis and what ELF score indicates low risk. What would happen to the patient in each case?
- ELF score < 9.5 = low risk so patient would be managed in primary care
- ELF score > 9.5 = high risk so patient would be refered to hepatology clinic
Describe the GI tract
- 7-10m continuous tube that runs from mouth to the anus
- Made up of many sections each with distinct anatomy and functions
Describe the anatomy of the GI tract
- Main organs are:
- Oesophagus
- Stomach
- Large intestine (Colon)
- Small intestine (Duodenum, Jejunum and Ileum)
- Acessory organs are:
- Liver
- Gallbladder
- Pancreas
What is a gastric ulcer?
- A break in the protective stomach mucosal lining
What is Helicobacter Pylori (H.Pylori) and how is it able to survive in the stomach?
- H.pylori is helix-shaped gram-negative bacteria
- It survives in the gastric acid by secreting urease which breaks down urea to form CO2 and ammonia
- Ammonia neutralises gastric acid allowing H.Pylori to proliferate and damage the mucosal layer in the stomach
How is H.Pylori able to be detected?
- Patient drinks a solution containing urea labelled with an uncommon isotope e.g. non-radioactive carbon-13.
- The detection of isotope-labelled carbon dioxide in exhaled breath indicates that the urea was broken down by urease-secreting H. pylori, present in the stomach.
What is Vitamin B12?
- Vitamin B12 (cobalamin) is a water soluble vitamin that has an essential role in the nervous system and the formation of red blood cells as a co-factor for DNA synthesis.
Describe the process of vitamin B12 absorption
- When dietary vitamin B12 enters the stomach it is bound to intrinsic factor (IF) by the parietal cells of the stomach
- IF-bound Vitamin B12 enters intestine and when it reaches terminal ileum it binds to mucosal cells on ileum membrane
- At this point Vitamin B12 dissociates from IF and is taken up by transcobalamin which takes it to the blood stream
What are some signs and symptoms of Vitamin B12 defieciency?
- Macrocytic anaemia (increased mean cell volume, MCV, decreased haemoglobin)
- Weakness and tiredness
- Pale skin
- Glossitis – inflammation of the tongue
- Nerve problems such as numbness or tingling (severe deficiency)
What is Coeliac disease?
- An autoimmune disorder, primarily affecting the small intestine.
- The disease results from immunological hypersensitivity to the ingested form of gluten (gliadin)
- When small intestine is exposed to gliadin there’s an inflammatory reaction leading to the shortening of the villi lining and villous atrophy
What is inflammatory bowel disease?
- Inflammatory bowel disease(IBD) encompasses two distinct autoimmune conditions of the GI tract:
- Ulcerative colitis - Diffuse inflammation affecting the mucosa of the colon only.
- Crohn’s disease - Patchy ulceration affecting any part of the GI tract and may extend through to the bowel
Inflammatory bowel disease (IBD) and Irritable Bowel syndrome (IBS) have some common symptoms, e.g. abdominal pain. What are some of the differences between IBS and IBD?
What is calprotectin and what is its mechanism of action?
- Calprotectin is a zinc and calcium binding protein loacted within the cytosol of neutrophils
- When there’s an inflammatory response in intestine neutrophils will migrate to lumen and will release calprotectin
- Neutrophils will also leak into bloodstream from intestine due intestinal damage
- This leads to Calprotectin being released into bloodstream which eventually ends up being excreted into faeces where it can be measured.
- Calprotectin will be present in faeces of person with IBD but NOT IBS because there;s no inflammatory response during IBS
