Liver Function and Testing Flashcards
1
Q
Liver
A
- Weight: ~1.2 – 1.6 kg
- 4 lobes
- Blood supply: hepatic artery (25%), portal vein (75%)
- Majority of blood from GI tract
- “First pickings” of absorbed nutrients
- Multifunctional: one of the most important metabolic organs in the mammalian body
- only human organ that can regenerate itself
2
Q
Processes that liver is involved in
A
- Waste management and treatment
- Recycling
- Production and storage of many essential molecules
3
Q
Liver in carbohydrate metabolism
A
- Major role in maintaining blood glucose levels
- Synthesis and storage of glucose units as glycogen (animal starch) during periods of carbohydrate availability
- During fasting glucose levels maintained by glycogenolysis
- Gluconeogenesis occurs mainly in the liver
4
Q
Liver in lipid metabolism
A
- Synthesis of almost all lipoproteins, phospholipids, cholesterol and endogenous triglycerides: malfunction can have severe cardiovascular consequences
- Breakdown products of cholesterol are excreted in bile
5
Q
Liver in protein synthesis
A
- Plasma proteins including special carrier proteins and coagulation factors are synthesised in hepatic cells
- Immunoglobulins are not synthesised in the liver
6
Q
Cofactors
A
- Vitamin D metabolism
- Vitamins A, D, E, K, B9 (folate) and B12 are stored in the liver along with copper and fats
- most important site of iron storage
7
Q
Liver in excretion and detoxification
A
- Detoxification of xenobiotics (drugs, toxins etc..)
- Conversion of ammonia to urea
- Conjugation of various compounds with ‘solubility-enhancing modifications’ such as glucuronation and sulfation to improve excretion: cholesterol, bilirubin, toxins
8
Q
What happens after senescence of RBCs
A
- erythrocytes, average life span: 120 days
- released Hb is split into globin (protein) and haem (Fe-containing porphyrin system)
9
Q
Steps in bilirubin metabolism
A
- Haem degraded to bilirubin via biliverdin by haem oxygenase and bilverdin reductase
- Bilirubin complexed with albumin and complex is transported to liver
- Hepatocytes take up bilirubin-albumin complex
- Conjugation of bilirubin with glucuronides to increase solubility and subsequent secretion in bile
- Bacterial breakdown to urobilinogens (excreted in faeces). ~20% urobilinogen reabsorbed and re-excreted in bile and urine
10
Q
Bacterial breakdown to urobilinogens
A
- Urobilinogen and stercobilinogen are colourless
- Bilirubin-derived pigments responsible for colour of urine (urobilin) and faeces (stercobilin)
- Pale stool can indicate problems with bilirubin metabolism
11
Q
Bilirubin - protective role
A
- Yellow pigment responsible for jaundice: potential cause of permanent brain damage or death in newborn babies
- 1987: publication of a study which demonstrated that bilirubin has antioxidant property in vitro
- Considerable evidence: minute amount (nanomolar) has important protective function, damage caused by highly reactive free radicals
- Inverse relationship between serum bilirubin concentration within the reference range and future risk of coronary artery disease (CAD)
12
Q
What is liver function testing used for?
A
- Assessing the most likely type of disease and possible underlying causes
- Determining the severity/stage of the disease
- Monitoring response to treatment
13
Q
Tests that can detect abnormalities in liver function
A
- Indirect measures: Total Bilirubin (and derivatives) levels in blood and urine, Total protein and albumin levels in plasma, Prothrombin time
- Measurement of enzymatic activity that has ‘leaked’ into the plasma due to hepatocellular damage, Alanine (ALT) and Aspartate (AST) aminotransferases
- Other enzymatic activities used to indicate abnormalities in liver function, Alkaline phosphatase (ALP), gamma-Glutamyl Transferase (GGT)
14
Q
Direct bilirubin (measurement in serum)
A
- primarily conjugated bilirubin
- all water soluble bilirubin
- formation of azidopyroles: add diazonium salt directly to sample. Will react with conjugated bilirubin
15
Q
Indirect bilirubin (measurement in serum)
A
- unconjugated bilirubin
- calculated as the difference (= Total–Direct)
16
Q
Total bilirubin (measurement in serum)
A
- sum of conjugated and unconjugated
- both forms, unconjugated bilirubin is solubilized by an accelerator (caffeine-sodium benzoate)
- formation of azidopyroles: add caffeine-sodium benzoate reagent followed by diazonium salt. All bilirubin will react
17
Q
What happens at the end of reaction (measurement of bilirubin in serum)
A
- Add ascorbic acid to stop the reaction
- Add alkaline tartrate to produce the blue-coloured form of the azidopyroles
- Measure absorbance = 600 nm
18
Q
Measurement of bilirubin in urine
A
- Same principle as for serum, but with a different diazonium salt
- Has to be done on fresh urine
- Only conjugated bilirubin ends up in the urine (unconjugated bilirubin is water insoluble)
- Test will detect about 3 mmol/L
- False positive found in patients on large doses of chlorpromazine
19
Q
Measurement of urobilinogen in urine
A
- p-dimethylaminobenzaldehyde and an acid buffer which reacts with urobilinogen
- Will detect urobilinogen in urine from some normal subjects
- False positives occur with p-aminosalicylic acid and some sulphonamides
20
Q
Measurement of protein
A
- Total protein of biological fluids: (serum, urine, cerebral spinal fluid (CSF))
- Biuret test: under alkaline conditions cupric ions (Cu2+) react with proteins with at least two peptide bonds
- Reagent contains Na+/K+-tartrate to form complex with the Cu2+ ions and maintain solubility in alkaline solution, iodide included as antioxidant
- absorbance of Cu+-protein complex at 540 nm directly proportional to concentration of protein in the sample
21
Q
Conditions of measurement of total protein
A
- Either serum or plasma may be used for biuret assay but serum is preferred
- fasting specimen is desirable (not essential) to decrease risk of lipaemia
- Haemolysis should be avoided
- Specimens that have been frozen and thawed should be thoroughly mixed before assay
22
Q
Properties of albumin
A
- Globular protein with a molecular mass of 66.3 kDa
- Normally: most abundant protein in plasma (~½ the protein mass)
- Levels range: 35-50 g/L
- Abnormally high levels are a consequence of dehydration
- Levels typically lower in hospital in-patients
- Synthesized primarily by the liver
23
Q
Functions of albumin
A
- Maintenance of colloidal osmotic pressure (COP) in both vascular and extravascular spaces, with continuous equilibrium between them (primary)
- Transport of a large number of compounds including free fatty acids, metallic ions, hormones, drugs and bilirubin
24
Q
Measurement of albumin
A
- Typical: automated dye-binding methods which use bromocresol green (BCG) or purple (BCP) dyes - great affinity for albumin
- absorbance of albumin-BCG complex is measured at 628 nm, is proportional to albumin concentration in the sample
- use of serum rather than plasma is recommended: overestimate albumin in the presence of fibrinogen and heparin
- Erroneous if overall serum protein pattern is abnormal
25
Q
a-Fetoprotein (AFP): Fetal Albumin
A
- Normally present in foetal serum and to lesser extent maternal serum and urine (clears fast)
- Abnormal levels can indicate birth defects
- Detection via antibody test
- Normally undetectable beyond infancy
- Elevated levels in males and non-pregnant females can indicate several tumours: Hepatocellular carcinoma, Metastases affecting the liver
26
Q
Measurement of prothrombin time
A
- Most coag factors are produced in the liver
- Measured in blood plasma from blood sample with added anticoagulant citrate (binds Ca2+)
- Coag is started by adding: excess Ca2+ to reverse the effect of the citrate, thromboplastin (factor III, degrades in sample, so needs to be added to start the reactions)
- Coag time is measured optically
27
Q
Measurement of Alanine Aminotransferase (ALT)
A
- ALT converts l-alanine to pyruvate. The latter can be detected via follow-up reactions
- Monitor decrease in absorbance at 340 nm
- Incubate sample with 2,4-dinitrophenylhydrazine for 20 min, stop reaction by adding a strong base
28
Q
Measurement of Aspartate Aminotransferase (AST)
A
- AST converts l-aspartate to oxaloacetate. The latter can be detected via follow-up reactions
- Monitor decrease in absorbance at 340 nm
- Incubate sample with 2,4-dinitrophenylhydrazine for 20 min, stop reaction by adding a strong base
29
Q
Measurement of Alkaline Phosphatase (ALP)
A
- ALP activity is determined by measuring rate of conversion of pNPP to pNP in presence of Mg2+ and Zn2+
- change in absorbance is measured at 410/480 nm (ideally 415 nm), directly proportional to ALP activity – yellow colour
30
Q
Measurement of gamma-Glutamyl Transferase (GGT)
A
- GGT catalyses transfer of g-glutamyl group from substrate, g-glutamyl-3-carboxy-4-nitroanilide to glycylglycine, yielding yellow-coloured 5-amino-2-nitrobenzoate
- change in absorbance is measured at 405 nm, directly proportional to GGT activity in the sample
31
Q
Non-laboratory tests
A
- Ultrasound: good for visualising large bile ducts, fatty liver, large masses; cheap, non-invasive; unable to detect inflammation/cirrhosis (unless advanced)
- ERCP (Endoscopic retrograde cholangio-pancreatography): can visualise smaller bile ducts, head of pancreas; expensive, invasive
- Liver Biopsy: can see hepatic pathology, hepatocytes; gold standard; expensive, invasive
32
Q
What is the underlying pathology detected with which test?
A
- Hepatocellular damage: leakage of particular enzymes into the extracellular fluid (plasma); Alanine (ALT) and Aspartate (AST) aminotransferases
- General impairment of metabolism: lack of albumin and coagulation factors (prothrombin time), bilirubin levels (not exclusive to liver function)
- Biliary tract involvement: increased levels of ALP and GGT, bilirubin levels
- Bilirubin levels generally only affected in the long-term
33
Q
Causes of Biliary Obstruction
A
- impaired bile secretion: cystic fibrosis, inflammation (sepsis, hepatitis), toxins, drugs (inhibit bile transport), hormones
- obstruction of bile flow: primary biliary cirrhosis, primary sclerosing cholangitis, gallstones, tumours (Ca head of pancreas, metastatic cancer, etc)
34
Q
Aminotransferases
A
- AST isn’t specific to hepatocyte damage, can also be raised due to myocardial infarction, ALT is specific to hepatocyte damage
- Leak out of damaged hepatocytes
- AST:ALT ratio sometimes useful (if >1 and total levels elevated, cause most likely not liver-related)
- Non-hepatic causes of elevated AST & ALT: coeliac disease, haemolysis, hyperthyroidism
35
Q
gamma-glutamyltransferase (GGT)
A
- Most sensitive indicator of hepatobiliary disease but is not specific
- Produced in hepatocytes and biliary epithelial cells but also pancreas, renal tubules and intestine
- Many drugs induce GGT synthesis causing elevated plasma levels despite normal liver function e.g. phenytoin, barbiturates, alcohol
- Also associated with CVD and other clinical conditions e.g. pancreatic disease, MI, COPD, renal failure, DM
- If other tests of hepatocellular damage are normal, a high level may suggest liver disease
36
Q
Jaundice/Hyperbilirubinaemia
A
- Cause: abnormal metabolism or retention of bilirubin
- External characteristic: brownish-yellow pigmentation of skin, sclera and mucous membranes
- Not exclusively associated with liver disease
- Not all liver diseases cause hyperbilirubinaemia
37
Q
Unconjugated Hyperbilirubinaemia
A
- Excess of bilirubin is unconjugated - Jaundice (in adults levels) rarely exceeds 100 mmol/L
- Not result of liver disease but due to either: haemolysis: increased bilirubin production, gilbert’s syndrome: decreased conjugation activity in liver (harmless in isolation), transient neonatal jaundice: liver enzymes not yet fully active
- Excess bilirubin exceeds capacity of liver to remove and conjugate the pigment
- More bilirubin is excreted in bile and urinary urobilinogen is increased
38
Q
Conjugated Hyperbilirubinaemia
A
- Leakage of bilirubin from liver cells or biliary system into bloodstream - normal excretion route is blocked
- Water-soluble conjugated bilirubin entering systemic circulation is excreted in urine - deep orange-brown colour
- Complete biliary obstruction - no bilirubin reaches gut, no urobilin is formed and stools are pale
- Separate measurement of conjugated and unconjugated bilirubin - assess relative contributions of defective conjugation and other causes
39
Q
Types of jaundice
A
- Prehepatic: increased bilirubin production, haemolysis
- Hepatic: defect in bilirubin metabolism (impaired uptake/conjugation/secretion), genetic defects, primary liver disease
- Posthepatic: defect in bilirubin excretion, bile duct obstruction, pancreatic head Ca
40
Q
Causes of prehepatic jaundice
A
- Causes of haemolytic jaundice: acute or chronic haemolytic anaemia, neonatal physiological jaundice
- Underlying causes: inherited RBC disorders, Vit B12 deficiency, Blood type (A, B, AB, 0) and rhesus factor (+/-) incompatibility, drugs
41
Q
Prehepatic jaundice
A
- Increased destruction of RCs brings larger load to liver than it can handle
- Additional bilirubin is conjugated and excreted into intestinal tract, increased amount of urobilinogen is formed in colon
- Some is reabsorbed and returned to liver
- Liver can’t pick up large amounts of urobilinogen - increased amounts excreted in urine
- determination of urobilinogen in urine and stool are useful in diagnosis of haemolytic anaemia
42
Q
Conjugation failure (hepatic jaundice)
A
- Crigler-Najjar disease: deficiency of UDP-glucuronyl transferase (less severe: Gilbert’s syndrome: decreased UDP-glucuronyl transferase activity)
- Neonatal physiological jaundice (transient)
43
Q
Post-conjugation failure (hepatic jaundice)
A
- Dubin-Johnson syndrome: mutation in multidrug transporter prevents transport of conjugated bilirubin into the bile
44
Q
Hepatic jaundice
A
- Viral, toxic hepatitis and cirrhosis involve overall damage, necrosis of liver cells
- Injured cells lose ability to remove and conjugate bilirubin = increase in unconjugated bilirubin
- damaged cells provide a path for leakage of bilirubin glucuronides back into the sinusoids = increase in conjugated bilirubin as well
- Not capable of removing all the urobilinogen from portal blood and more reaches the general circulation
- Kidney excretes excess urobilinogen and conjugated bilirubin into the urine in appreciable quantities
45
Q
Post-hepatic jaundice
A
- obstruction of common bile duct by: stones, neoplasms, spasms or strictures
- Produces a condition known as obstructive jaundice
- Conjugated bilirubin can’t reach intestine, so no urobilinogen is produced for recirculation to liver or excretion in stool
- Faeces vary in colour from light brown to chalky white
- Urine doesn’t contain urobilinogen but does contain appreciable amounts of conjugated bilirubin
- blood contains increased amounts of unconjugated and conjugated bilirubin, caused by regurgitation and liver cell impairment
46
Q
Obstructive Cholestasis
A
- Typically accompanied by elevated ALP
- Can cause secondary hepatic damage
47
Q
Liver disease
A
- Severe consequences of liver diseases due to central role in many vital processes
- Eg: Hepatic encephalopathy due to ammonia build-up in blood (ammonia detoxification is impaired)
- liver is a resilient organ: damage needs to be substantial in order for clinical abnormalities to manifest themselves
48
Q
Most common diseases affecting the liver
A
- Hepatitis with hepatocyte damage
- Cirrhosis
- Tumours (often secondary)
49
Q
Toxic hepatitis
A
- Damage by external toxins: paracetamol (acute overdose), carbon tetrachloride, alcohol
- Long-term alcohol abuse can also cause cirrhosis
50
Q
Causative agents of viral hepatitis
A
- Hepatitis A, B, C, D, E and G
- Epstein-Barr
- cytomegalovirus
51
Q
Viral hepatitis (I)
A
- B, C and D: contracted through blood of infected person (unsafe injection or unscreened blood transfusion)
- B and C also through unprotected sex
- D only infects persons infected with B
- A and E typically transmitted via contaminated water or food - closely associated with poor sanitation and poor personal hygiene
- At present effective vaccines are available for all hepatitis types except C
52
Q
Viral hepatitis (II)
A
- severe cases of hep B or C hepatic failure may develop
- some cases of hep B, persistent antigenaemia occurs, may lead to development of chronic liver disease
- Hep A infection never leads to chronic disease - a full recovery within 2 months with no permanent liver damage
- 20% of patients with hep C will develop cirrhosis over a period of 20-30 years – liver transplant
- Chronic hep C infection is also associated with increased risk of developing liver cancer
53
Q
Viral hepatitis diagnosis
A
- hepatocellular injury is due to host immune reactions against virus infected cells
- During early acute phase plasma aminotransferases rise, with concentrations >50X the normal reference limit
- As acute illness subsides aminotransferase activity returns to normal
- Aminotransferases may be moderately elevated for longer in chronic cases
- Plasma bilirubin levels rise more slowly, peaking at 10-20X the reference limit
- ALP and GGT activities are generally only mildly elevated
54
Q
Cirrhosis
A
- Liver fibrosis - the accumulation of tough, fibrous scar tissue in liver
- Over time results in cirrhosis of the liver - functional capacity of liver becomes disrupted
- Further serious complications may include portal hypertension, liver failure and liver cancer
- Irreversible
55
Q
Causes of cirrhosis
A
- chronic excessive alcohol intake
- autoimmune disease
- persistence of hepatitis B virus
- metabolic diseases e.g. Wilson’s disease
56
Q
Cirrhosis diagnosis
A
- detecting and assessing liver fibrosis: trans-abdominal needle biopsy of the liver
- Painful and hazardous and is time-consuming and subject to inter-observer variability = poor reference standard
57
Q
Indirect markers for fibrosis
A
- Markers released into the blood due to liver inflammation (ALT and AST)
- Markers synthesised, regulated or excreted by the liver (clotting factors, cholesterol, bilirubin)
- Processes that become disturbed by impaired liver function (insulin resistance)
