Lab Investigation of Liver and GI Tract Disease Flashcards
Describe the structure of the liver.
The liver is the largest organ in the body and is located in the upper right quadrant of the abdomen. It weighs approximately 22 g/kg of body weight (1.5 kg in an average 70 kg male).
The liver has a dual blood supply – 2/3 comes from the gut via the portal vein and 1/3 from the hepatic artery (rich in oxygen). Blood leaves the liver through the hepatic veins.
The liver has two lobes which are each composed of multiple liver lobules. These consist of plates of hepatocytes radiating from a central vein, which carries blood from the liver.
Each lobule can be further divided into liver acini, the smallest functional unit of the liver.
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.
List 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 common disease processes affecting the liver?
HEPATITIS:
- Damage to hepatocytes
CIRRHOSIS:
- Increased fibrosis
- Liver shrinkage
- Decreased hepatocellular function
- Obstruction of bile flow
TUMOURS:
- Frequently secondary: colon, stomach, bronchus
How would you biochemically assess liver function?
You could perform biochemical tests (LFTs):
- Insensitive indicators of liver function,
sensitive indicators of liver damage
- Look for pattern of results - a single result rarely provides a diagnosis on its own
Interpretation must be performed within the context of the patient’s risk factors, symptoms, medications, current condition/illness and physical findings.
What are liver function tests (LFTs) used for?
LFTS are NOT diagnostic, but can be 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
What does a LFT include?
The standard LFT profile at St George’s Hospital includes:
- Total Bilirubin
- Alanine aminotransferase (ALT)
- Alkaline phosphatase (ALP)
- Albumin
Describe jaundice.
Jaundice describes the yellow discolouration of tissue due to bilirubin deposition. An increase in total bilirubin is termed hyperbilirubinaemia.
Clinical jaundice may not be evident until the serum/plasma bilirubin concentration is 2x the upper reference of normal, >50 μmol/L.
Causes include:
- HAEMOLYSIS (increased bilirubin production) – acquired autoimmune haemolytic jaundice, drug induced and spherocytosis
- HEPATOCELLULAR DAMAGE (impaired bilirubin metabolism) – toxins or infections
- CHOLESTASIS (decreased bilirubin excretion) – Cirrhosis, tumour or gallstones
Describe the lab investigations of bilirubin in plasma/blood.
Bilirubin is measured in serum/plasma samples as:
- TOTAL BILIRUBIN - Unconjugated and conjugated bilirubin (& delta bilirubin)
- DIRECT – Conjugated bilirubin (& delta bilirubin)
- INDIRECT – Unconjugated bilirubin (calculated)
Delta bilirubin is formed by the irreversible covalent addition of bilirubin to albumin that occurs in the presence of prolonged conjugated hyperbilirubinaemia.
Describe the lab investigations of bilirubin in urine.
Bilirubin can be measured in urine using a simple dipstick. It uses a similar diazo method.
As unconjugated bilirubin is protein bound it is not normally found in urine. The presence of bilirubin in the urine therefore indicates the presence of conjugated hyperbilirubinaemia.
Excess conjugated bilirubin will darken the urine. This is seen in cases of hepatitis or impaired flow of bile in patients with biliary obstruction.
What is the significance of urobilinogen in the urine?
If urobilinogen is present in the urine it demonstrates that bilirubin is reaching the gut. It can be detected by a urine dipstick.
Excess urobilinogen in the urine may indicate liver disease such as viral hepatitis and cirrhosis or haemolytic conditions associated with increased red cell destruction.
Describe Alanine Aminotransferase (ALT).
ALT is a intracellular cytoplasmic enzyme that catalyses the transfer of an amino group from alanine to α-ketoglutarate. It’s a key enzyme in gluconeogenesis, with a ~47 hour half-life in plasma.
ALT is the most specific marker for liver injury although it is also expressed by the kidneys and cardiac and skeletal muscle.
ALT is used to identify liver damage arising from hepatocyte inflammation or necrosis.
Values >20x the upper limit of normal (ULN) may occur with severe liver damage. Small increases (<5x ULN) may occur in cholestasis due to secondary damage to hepatocytes.
Describe Aspartate Aminotransferase (AST).
AST is a intracellular cytoplasmic and mitochondrial enzyme that catalyses the transfer of an amino group from aspartate to α-ketoglutarate. It has a half-life of ~17 hours in plasma.
AST is less liver specific than ALT and there is little use in measuring both enzymes.
The only indication for measuring ALT and AST is to determine the AST:ALT ratio:
<0.8 – suggestive of non-alcoholic fatty liver disease (NAFLD);
>1.5 – alcoholic liver disease (ALD).
AST is assayed using the same method as ALT, replacing alanine with aspartate and producing oxaloacetate rather than pyruvate.
Describe Alkaline Phosphatase (ALP).
ALP is a membrane bound glycoprotein enzyme that removes phosphate groups from proteins and nucleic acids.
It has maximum catalytic activity at pH 9-10.5.
ALP isoenzymes are found in a number of tissues. The greatest concentrations are found in bone, liver, (biliary canaliculi), intestine and placenta.
ALP is of major value in the diagnosis of cholestatic disease along with GGT as cholestasis stimulates enhanced synthesis of liver ALP.
ALP is elevated in children and correlates well with the rate of bone growth (child-specific reference ranges). Also increased during pregnancy due to an increase in placental ALP.
The source of an elevated ALP can be determined by gel electrophoresis. It is possible to separate ALP isoenzymes into liver, bone, and intestinal fractions.
The placental isoenzyme of ALP can be identified as it is heat stable at 65°C for 10 minutes, unlike the other isoenzymes.
Describe Gamma Glutamyl Transferase (GGT).
GGT is a membrane bound enzyme that transfers the gamma glutamyl group from peptides such as glutathione to other peptides and to L-amino acids.
A relatively specific marker for liver injury found on the canalicular membrane of hepatocytes, but also found on the cell membranes of other tissues.
Plasma GGT is elevated in response to consumption of alcohol and other drugs (eg. phenytoin, barbiturates)
GGT does not generally provide more information than the routine LFT results but it can be useful in determining the origin of an unexplained raised ALP:
Interpretation of ALP and GGT results:
- ↑ ALP and ↑ GGT – suggestive of hepatic cause (cholestasis)
- ↑ ALP and N GGT – suggestive of bone source of ALP
- N ALP and ↑ GGT – suggestive of excess alcohol intake
Describe Albumin.
Albumin has a half-life of around 20 days and the liver synthesises around 12 g every day.
Albumin is an essential plasma protein and has many roles including maintaining the plasma oncotic pressure and binding several hormones, drugs, anions and fatty acids.
Albumin is only a crude indicator of the synthetic capacity of the liver due to its long half-life and because plasma albumin levels can decrease due to renal or GI losses, and also decrease during the acute phase response.
Use of dye binding assay for albumin assay at St George’s Hospital.
At a pH of 5.2-6.8, bromocresol purple binds with albumin, causing an change in absorbance at 600 nm.
