Author(s): Rebecca W. Van Dyke, M.D., 2012
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M2 GI Sequence
Liver Tests: Use and
Interpretation
Rebecca W. Van Dyke, MD
Winter 2012
Learning Objectives
•
A.
General: Understand the laboratory tests that are used in the clinical
approach to liver disease and the pattern of abnormalities that occur in specific
forms of liver injury.
–
–
–
•
•
B.
–
–
–
1. When do we suspect a patient has liver disease? What tests can be used to accept or deny the
presence of liver disease?
2. Can we define the type of liver disease the patient has by analyzing the results of the liver tests?
3. How much functional liver tissue is present in a patient?
Specific:
1. Be able to interpret panels of biochemical liver tests in terms of general type of liver disease,
chronicity and severity.
2. Be able to construct a differential diagnosis for different patterns of liver test results.
3. Be able to identify potential problems in interpreting liver tests.
Industry Relationship Disclosures
Industry Supported Research and
Outside Relationships
• None
How Do We Tell Someone Has Liver
Disease?
Clues that may lead to a suspicion of liver disease:
Nonspecific
More specific
(late findings)
Anorexia
Fatigue
Nausea
Vomiting
Mental confusion
Jaundice (“yellow eyes”)
Dark urine (“coca-cola” urine)
Abdominal swelling; ascites
Peripheral edema; leg swelling
Unfortunately none of these are specific markers of liver disease
and for many patients these are very late findings.
Purpose of Liver Tests
1. Screen for clues to the presence of liver injury/disease
liver cell injury
bile flow/cholestasis
2. Quantitate degree of liver function/dysfunction
quantitative liver tests
3. Diagnose general type of liver disease
pattern of liver test abnormalities
4. Diagnosis of specific liver disease
disease-specific tests such as serology for
viral hepatitis
Tests of Liver Cell Injury/Death
Transaminases
Alanine amino transferase (ALT)
Aspartate amino transfersase (AST)
Transaminases are enzymes that catalyze the transfer of aamino groups from amino acids to a-keto acids.
These enzymes are important in gluconeogenesis.
ALT (alanine aminotransferase)
alanine
+
ketoglutarate
pyruvic acid
+
glutamate
AST (aspartate aminotransferase)
aspartate
+
ketoglutarate
oxaloacetic acid
+
glutamate
Transaminases:
AST(SGOT)
ALT(SGPT)
Many tissues
Liver only
Cytosol/mitochondria
Cytosol
Normal blood levels:
20-70 IU/liter (depending on method)
Some AST/ALT release occurs normally
Require pyridoxal 5’-phosphate as an essential cofactor
Multi-channel Automated Analysis of Enzymes in Blood
lamp
l
Patient
serum
ALT
+
substrate
Substrate
Colored
product
l
Photodetector
Absorbance
converted to
enzyme activity
Release of AST/ALT from Liver Cells
during Acute Hepatocellular Injury
AST
ALT
Transaminases
Why do we used these enzymes to indicate liver damage?
1. Convenient to measure
2. Present in liver cells in large amounts
3. Direct release of enzymes into blood through
fenestrated endothelium allows rapid
“quantitative” assessment of ongoing
hepatocyte necrosis
4. Blood level roughly proportional to the number
of hepatocytes that died recently (hours-days)
Ischemic infarction: how high would AST/ALT go?
Transaminases
Special considerations:
1. AST is also present in other tissues
(muscle, brain, kidney, intestine).
ALT is more specific for liver.
2. Even very mild liver abnormalities can cause
slightly elevated AST/ALT
- for example, mild fatty liver.
Fatty Liver:
Most Common Cause of Mildly Increased AST/ALT
(~1.5-3x upper limit of normal)
Transaminases
Problems with using transaminases to assess liver injury:
1. Only assess injury over the past 1-2 days as enzymes
are cleared efficiently from blood by RES
2. May not accurately assess hepatocyte death from
apoptosis
3. Magnitude of elevation does not necessarily correlate
with extent of liver function or dysfunction at the
present time or in the future.
AST and ALT = rate of destruction of hepatocytes
Liver function = number of functional hepatocytes left
Not all liver abnormalities cause liver cell
death: simple liver cyst with
normal liver tests
Transaminases and Alcoholic Liver
Disease: A Twist
Pyridoxal 5’-phosphate (P5P) deficiency:

AST and ALT require P5P (vit. B-6) as an enzymatic
cofactor

Alcoholics are often deficient in P5P as their major
calorie source is alcohol

P5P deficiency results in lower synthesis of AST/ALT
(less in hepatocytes) and very low enzymatic activity
(ALT worse than AST)

Less AST/ALT released into blood and it isn’t
measured by lab assays
Transaminases and Alcoholic Liver
Disease
Further: Mitochondrial AST and alcohol

Alcohol shifts mAST from mitochondria to plasma
membrane where it readily enters blood – thus AST
easier to remove from hepatocytes.
Therefore: AST>>ALT is released into blood
from damaged hepatocytes
AND
both AST/ALT enzymatic activities in blood
are lower than expected from the extent of liver
damage/dysfunction.
AST/ALT and Pyridoxal 5’ Phosphate
AST is Affected Less than ALT so AST>ALT
P5’P
Numerous active enzymes with P5’P
Few inactive enzymes without P5’P
Poorly measured by lab assays
Tests of Cholestasis/Reduced Bile Flow
Enzymes released as a
consequence of decreased
bile flow
Alkaline phosphatase
or
5’-nucleotidase
Leucine aminopeptidase
g-glutamyl transpeptidase
Accumulation in liver/blood
of substances normally
excreted in bile
Bilirubin
Bile salts
Alkaline Phosphatase:
Location at Canalicular (Apical) Membrane
Alkaline phosphatase
Origin of enzyme and mechanism of increase in
cholestatic liver disease:
1. Apical membrane of hepatocyte and bile duct cells
2. Very sensitive to any changes in bile flow,
obstruction of large or small bile ducts.
3. Amplified by bile acid retention
4. Easily released into blood as it is a GPI-anchored
protein solubilized from membrane by
detergents (bile acids)
Easily measured spectrophotometrically
Purpose: ? Detoxifies lipopolysaccharide (LPS) from bacteria
Bile Acids
(Bile Salts) such as
Taurocholate
stimulate production
of alkaline
phosphatase
molecules.
Normal (A)
and
(B) Blebbed
Hepatocytes
Bile acid-induced injury
Release of GPI-Anchored Proteins From
Liver During Cholestasis
GPI-Anchored Proteins
Alkaline
Phosphatase
5’ Nucleotidase
GGTP
Hepatocyte
Bile canaliculus
Blood
Alkaline Phosphatase in Various Liver Diseases
1500
Degree of elevation of AP is highly variable
depending on duration and extent of
cholestasis and other unknown factors.
1000
500
Serum
Enzyme
Level
(IU/ml)
200
100
Long-standing
bile duct
obstruction
Acute bile
duct
obstruction
Mild early
partial
bile duct
obstruction
Hepatocellular
disease
Alkaline Phosphatase
Interpretation of elevated levels:
1. Cholestasis (especially in extrahepatic
obstruction
2. Infiltrative diseases (granulomas)
3. Neoplastic disease infiltrating liver
Sensitive test as will go up if only some small ducts are
obstructed and/or if there is only partial obstruction of
major ducts.
Disadvantages:
Not completely specific because of isoenzymes
in other organs (bone, intestine, placenta)
Ex: bone disease, intestinal obstruction, pregnancy.
Serum Bilirubin (Bile Acids)
Rationale:
Liver is virtually the only mechanism for excretion
Cholestasis from any cause results in “back-up”
of these compounds in blood
Interpretation:
Cholestasis: extrahepatic or intrahepatic
Disadvantages:
Does not distinguish hepatocellular disease,
in which hepatocytes don’t make bile,
from bile duct obstruction
Bilirubin
An organic anion
The byproduct of heme breakdown
In mammals bilirubin must be conjugated to
glucuronic acid and excreted in bile
Blood levels go up if any steps in production or
hepatocyte excretion are altered. However
obstruction at the level of bile ducts must be
complete or virtually complete for bilirubin
levels in blood to change
Hepatic Bilirubin Transport
SER
RBC
breakdown
in RES
UDP-glucuronide
+
Unconj BR
Conj
BR
Unconj
Bilirubin
Unconj BR
Bile
Canaliculus
Conj
BR
MRP-2:
Multispecific organic
anion transporter
Conj
BR
ATP
Blood
Hepatocyte
Conjugated bilirubin
Glutathione S-conjugates
other organic anions
Hepatic Bilirubin Transport and Mechanisms
of Hyperbilirubinemia
Gilbert's syndrome (mild)
Crigler-Najjar syndrome (severe)
SER
Hemolysis
Unconj
Bilirubin
Bile
Canaliculus
Unconj BR
Conj
BR
Multispecific organic
anion transporter
Conj
BR
ATP
Conjugated bilirubin
Glutathione S-conjugates
other organic anions
Blood
Hepatocyte
Dubin-Johnson syndrome
Rotor's syndrome
?estrogen/cyclosporin
Interpretation of Elevated Serum Bilirubin
Conjugated hyperbilirubinemia:
BR reached liver and was conjugated but not excreted in bile
1. Cholestasis/biliary obstruction (must be essentially complete)
2. Hepatocellular damage (collateral damage to all liver functions)
bile formation impaired >> conjugation impaired
3. Rare disorders of canalicular secretion of conjugated bilirubin
Unconjugated hyperbilirubinemia:
BR didn't reach liver efficiently or wasn't conjugated
1. Massive overproduction - acute hemolysis
2. Impaired conjugation
common:
Gilbert's syndrome (mild)
rare:
Crigler-Najjar syndrome (severe)
Further: Bilirubin Undergoes Non-Enzymatic Reaction
with Albumin
Why is Bili-Albumin of Clinical
Interest?
• Not of interest during liver disease as this form of
bilirubin is measured as conjugated bilirubin.
• However, after resolution of cholestasis/liver disease,
bili-albumin is cleared like albumin
– Albumin half-life:
– Conj. bilirubin half-life:
–
several weeks
hours to days
• Thus resolution of jaundice is often SLOW compared
to improvement of other liver functions.
Purpose of Liver Tests
1. Screen for clues to the presence of liver injury/disease
liver cell injury
bile flow/cholestasis
2. Quantitate degree of liver function/dysfunction
quantitative liver tests
3. Diagnose general type of liver disease
pattern of liver test abnormalities
4. Diagnosis of specific liver disease
disease-specific tests such as serology for
viral hepatitis
TESTS OF LIVER FUNCTION
Blood Level
Production
Albumin
Clotting factors
Blood Level
Bilirubin
Bile Acids
Elimination
Metabolism
14C-Aminopyrine
Metabolites
14CO
2
Albumin
• Rationale
– Liver is the sole source
• Interpretation of Decreased Level
– Decreased liver production
– Increased renal/GI loss (nephrotic syndrome;
protein losing enteropathy in inflammatory bowel
disease
– Protein malnutrition
• Disadvantages
– Prolonged half-life
– No unique interpretation
Prothrombin Time
• Rationale
– Liver is sole source of vitamin K-dependent
clotting factors, including those critical for PT
– Factor VII has very short half-life (hours)
• Interpretation of Increased PT
– Hepatocyte protein synthesis impaired
– Vitamin K deficiency/Coumadin therapy
– Disseminated intravascular coagulopathy
• Advantages
– Rapidly reflects changes in liver function
Interpretation of Abnormal
Albumin/Prothrombin Time
Liver markedly diseased - reserve function gone
Albumin:
monitors slow changes in liver function
(months to years)
reflects long-term liver dysfunction
Protime:
monitors rapid changes in liver function
(hours to days/weeks)
reflects either short or long-term liver
dysfunction
Other Clues to Globally Impaired Liver Function
Bilirubin:
goes up with any disease that globally impairs
liver function (OR blocks bile flow)
Glucose:
hypoglycemia (late finding; indicates
very poor liver function)
BUN:
low BUN is a late and poorly specific finding
in liver dysfunction due to poor urea
synthesis
Purpose of Liver Tests
1. Screen for clues to the presence of liver injury/disease
liver cell injury
bile flow/cholestasis
2. Quantitate degree of liver function/dysfunction
quantitative liver tests
3. Diagnose general type of liver disease
pattern of liver test abnormalities
4. Diagnosis of specific liver disease
disease-specific tests such as serology for
viral hepatitis
Interpretation of Liver Tests
A. Consider nonhepatic causes of abnormal liver tests
B. Examine the pattern of liver test abnormalities to
categorize liver disease:
1. cholestatic
versus
hepatocellular
2. acute
versus
chronic
3. decompensated
function
versus
mild functional
impairment
Patterns of Abnormal Liver Tests
Hepatocellular
Cholestasis
Major:
AST/ALT
Alkaline Phosphatase
Minor:
Bilirubin
PT/albumin
Bilirubin
AST/ALT
PT (Vit K deficiency)
Clues to Acute vs Chronic Liver Disease
Abnormalities of PT versus albumin
Known duration of abnormal liver tests
History of exposure to potential causative agents
Clinical signs of consequences of long-standing liver disease
Tempo of subsequent changes in AST/ALT, bilirubin, PT
chronic tends to change slowly
acute tends to change quickly
Clues to Severity of Liver Dysfunction
Prothrombin time
Albumin
(Bilirubin, glucose)
(Clinical signs of consequences
of severe liver dysfunction such
as hepatic encephalopathy)
Clues to severity
of liver damage
and, potentially, to
severity of liver
dysfunction may
come from the
temporal sequence
of changes in
readily available liver
tests.
For example, rapid
fall in AST/ALT
in severe hepatitis
may not be a good
sign.
Purpose of Liver Tests
1. Screen for clues to the presence of liver injury/disease
liver cell injury
bile flow/cholestasis
2. Quantitate degree of liver function/dysfunction
quantitative liver tests
3. Diagnose general type of liver disease
pattern of liver test abnormalities
4. Diagnosis of specific liver disease
disease-specific tests such as serology for
viral hepatitis
These are discussed in future lectures
Summary
• Use biochemical tests to assess
–
–
–
–
Presence of liver disease
General type of liver disease
Sense of severity of liver dysfunction
Sense of acute versus chronic disease
• Use liver tests with other data to work
through differential diagnosis
– See algorithms in syllabus and textbook
A variety of cases are provided in your
syllabus to allow practice in analyzing
liver tests.