Introduction
- AST stands for Aspartate Aminotransferase.
- AST is also called SGOT (Serum Glutamate Oxaloacetate Transaminase).
- It is an intracellular enzyme involved in amino acid metabolism.
- AST belongs to the transferase group of enzymes.
- It catalyzes transfer of amino group from aspartate to alpha-ketoglutarate.
- This reaction forms oxaloacetate and glutamate.
- AST is widely distributed in body tissues.
- Highest concentration is found in heart, liver, skeletal muscle, kidney, and red blood cells.
- Because AST is present in many tissues, it is less liver-specific than ALT.
- AST estimation is commonly performed in liver function testing and cardiac enzyme studies.
- Raised AST indicates tissue injury or cell damage.
- AST is usually interpreted together with ALT for better diagnosis.
- In liver disease, AST helps assess hepatocellular injury.
- In muscle and cardiac disorders, AST may also rise significantly.
Principle
- AST estimation is based on IFCC kinetic enzymatic method.
- The reaction occurs in two stages.
Primary Reaction
- AST catalyzes transfer of amino group from L-aspartate to 2-oxoglutarate.
- Oxaloacetate and L-glutamate are produced.
Reaction
L-Aspartate + 2-Oxoglutarate → Oxaloacetate + L-Glutamate
Secondary Reaction
- Oxaloacetate reacts with NADH in presence of Malate Dehydrogenase (MDH).
- Oxaloacetate is reduced to malate.
- NADH is oxidized to NAD⁺.
Reaction
Oxaloacetate + NADH → Malate + NAD⁺
Additional Reaction
- LDH in reagent removes endogenous pyruvate interference.
Principle of Measurement
- NADH absorbs ultraviolet light at 340 nm.
- NAD⁺ does not absorb at this wavelength.
- During reaction NADH decreases continuously.
- This causes fall in absorbance.
- Rate of absorbance decrease per minute is proportional to AST activity.
Specimen
Type of Sample
- Serum is preferred specimen.
- Plasma may also be used.
- EDTA plasma is acceptable.
- Heparinized plasma can also be used.
Precautions
- Use non-hemolyzed serum only.
- Hemolysis must be avoided because red blood cells contain high AST.
- Hemolyzed sample gives falsely elevated result.
Sample Handling
- Fresh sample preferred.
- Separate serum early.
- Avoid contamination.
Storage
- Stable for 3 days at 2–8°C
- Stable for several months at −20°C
Reagents
Reagent 1 (Buffer Reagent)
- Tris buffer (pH 7.8)
- L-aspartate
- LDH enzyme
- MDH enzyme
Reagent 2 (Substrate Reagent)
- CAPSO
- 2-oxoglutarate
- NADH
Role of Reagents
- Tris Buffer – Maintains optimal pH
- L-Aspartate – Substrate for AST reaction
- MDH – Catalyzes secondary reaction
- LDH – Removes endogenous pyruvate interference
- NADH – Indicator molecule for kinetic measurement
Materials Required
- Test tubes
- Micropipette
- Pipette tips
- Semi-auto analyzer
- Spectrophotometer
- Cuvette
- Water bath
- Timer
- AST reagent kit
Procedure
Two Reagent Method
| Components | Quantity |
|---|---|
| Reagent 1 | 1000 µL |
| Sample | 100 µL |
First Incubation
- Mix properly
- Incubate at 37°C for 5 minutes
Then Add
| Components | Quantity |
|---|---|
| Reagent 2 | 250 µL |
Second Incubation
- Mix again
- Incubate at 37°C for 1 minute
Reading
- Read absorbance at 340 nm
- Record absorbance at 1 minute, 2 minute, and 3 minute
- Calculate average ΔA/min
Calculation
Formula
AST (U/L) = ΔA/min × Factor
Factor at 37°C and 340 nm
1745
Example
- Absorbance at 1 minute = 0.700
- Absorbance at 2 minute = 0.680
- Absorbance at 3 minute = 0.660
ΔA/min
- Difference = 0.020
Final Calculation
- AST = 0.020 × 1745
- AST = 34.9 U/L
Normal Reference Values
| Group | Normal Value |
|---|---|
| Men | Up to 35 U/L |
| Women | Up to 31 U/L |
Clinical Significance
- AST is an important intracellular enzyme released into blood when cells are damaged.
- Because AST is present in many tissues, an increased serum AST level indicates tissue injury but does not always specifically indicate liver disease.
- The major tissues containing high AST activity are heart, liver, skeletal muscle, kidney, brain, and erythrocytes.
- Therefore AST must always be interpreted together with clinical findings and other laboratory parameters.
Significance in Liver Disease
- AST rises when hepatocytes are damaged and enzyme leaks into circulation.
- In acute viral hepatitis, AST may increase several times above normal.
- In severe hepatitis, values may rise 10 to 20 times normal level.
- AST elevation often appears before jaundice develops.
- In toxic hepatitis, AST rises rapidly because of sudden hepatocellular necrosis.
- In drug-induced liver injury, AST may increase depending on severity of liver damage.
- In cirrhosis, AST is usually moderately elevated because chronic destruction of liver cells occurs slowly.
- In obstructive jaundice, AST may rise mildly to moderately.
- In fatty liver disease, mild AST elevation is common.
- In chronic hepatitis, persistent elevation suggests ongoing liver injury.
- AST is usually less elevated than ALT in pure hepatocellular injury.
AST in Alcoholic Liver Disease
- AST is particularly important in alcoholic liver disease.
- In alcohol-induced liver injury, AST often becomes higher than ALT.
- The AST/ALT ratio greater than 2 strongly suggests alcoholic hepatitis.
- This occurs because alcohol damages mitochondrial AST-rich liver cells and reduces ALT synthesis.
AST in Myocardial Infarction
- AST historically was one of the first cardiac enzymes used to diagnose myocardial infarction.
- In acute myocardial infarction, AST begins to rise within 6–8 hours after chest pain.
- Peak level occurs at 24–36 hours.
- It returns to normal within 3–5 days.
- Although troponin is now preferred, AST still indicates myocardial cell injury.
- AST rise in cardiac disease occurs because damaged cardiac muscle releases intracellular enzyme.
AST in Skeletal Muscle Disease
- Skeletal muscle contains large amounts of AST.
- Muscle injury can produce significant AST elevation.
- Increased AST occurs in:
- muscular dystrophy
- polymyositis
- severe exercise
- muscle trauma
- intramuscular injections
- crush injury
- In muscle disorders, AST may rise without liver disease.
AST in Hemolysis
- Red blood cells contain AST.
- Hemolysis during sample collection can falsely elevate AST result.
- Therefore non-hemolyzed serum is essential for correct interpretation.
AST in Kidney Disease
- AST may rise in severe renal tissue injury.
- Severe renal infarction can produce elevated AST.
- Kidney disease alone usually causes only mild elevation.
AST in Pancreatic Disease
- Acute pancreatitis may produce moderate AST increase.
- This occurs due to associated tissue injury and inflammation.
AST in Shock and Hypoxia
- Severe circulatory failure causes tissue hypoxia.
- Hypoxia damages liver and muscle cells.
- AST rises in shock because of generalized tissue injury.
AST in Burns and Trauma
- Extensive burns release AST from damaged tissues.
- Major trauma also causes AST elevation.
- Higher values indicate greater tissue destruction.
Diagnostic Importance of AST with ALT
- AST alone cannot identify exact organ source.
- ALT is more liver specific.
- Therefore AST is interpreted together with ALT.
Common Interpretation Pattern
AST > ALT
- alcoholic liver disease
- muscle injury
- myocardial injury
ALT > AST
- acute viral hepatitis
- hepatocellular liver injury
AST/ALT Ratio Clinical Use
- Ratio less than 1 usually suggests acute viral hepatitis.
- Ratio greater than 2 suggests alcoholic liver disease.
- Ratio near 1 may occur in chronic liver disease.
Prognostic Importance
- Very high AST indicates severe tissue destruction.
- Rapid fall after acute elevation may suggest recovery.
- Persistent elevation indicates ongoing injury.
Clinical Use in Monitoring
- Monitoring liver disease progression
- Assessing treatment response
- Detecting drug toxicity
- Evaluating muscle disorders
- Supporting diagnosis of cardiac injury
