TEST DIRECTORY

Aspartate Aminotransferase

Alternative Names: AST; GOT; SGOT; Transaminase, SGOT

Test Code: ASTL

Clinical Significance: The enzyme AST is widely distributed in tissue, primarily in the liver, cardiac muscle, skeletal muscle, kidney, brain, and erythrocytes. AST catalyzes the transfer of amino groups from L-aspartate to α-ketoglutarate, resulting in L-glutamate and oxaloacetate. This is a critical process of the tricarboxylic acid cycle, in which the coenzyme pyridoxal phosphate (also known as pyridoxal-5-phosphate or active vitamin B6) is required. In particular, AST is vital for aerobic glycolysis. AST exists in human tissues as two distinct isoenzymes, one located in the cytoplasm (c-AST), and the other in mitochondria (m-AST), which differ in amino acid composition and immunochemical and kinetic properties. In healthy individuals, the circulating AST levels consist mainly of cytoplasmic AST, originating from cytoplasmic leakage, on the other side, mitochondrial AST activity in serum shows a marked increase in patients with extensive liver cell degeneration and necrosis. Although AST activity is important in all cells with high metabolic activity, it is more relevant for liver and muscle cells.

Primarily, AST is a marker of hepatocellular injury. Measurement of AST activity is therefore used for the diagnosis of hepatic diseases such as acute and ischemic hepatopathy, suspected malignant infiltration, and cholestasis. Although alanine aminotransferase (ALT) is considered a more specific indicator of liver disease, the concentration of AST may be a more sensitive indicator of liver injury in conditions such as alcohol-related liver disease and in some cases autoimmune hepatitis. Several international guidelines recommend AST testing for monitoring chronic hepatitis status and progression.
Non-liver causes for increases in AST include damage to cardiac or skeletal muscle cells and hemolysis. Serum elevation of AST without elevation of ALT is suggestive cardiac or muscle disease. In patients undergoing renal dialysis or those with vitamin B6 deficiency, serum AST may be decreased. AST serum levels can be affected by age, gender, alcohol consumption, body mass index, dietary and living habits, nutrition, metabolic status, and drug treatment, among other factors.
In patients with vitamin B6 deficiency (insufficient endogenous pyridoxal phosphate), serum aminotransferase activity may be decreased. The addition of pyridoxal phosphate to this assay causes an increase in aminotransferase activity (activation higher for AST than for ALT) and prevents falsely low aminotransferase test results in these samples.

Turnaround Time: Typically, 24 hours

Preferred Specimen Type: Serum

Alternative Specimen Type: Lithium heparin plasma; K2 EDTA plasma; K3 EDTA plasma

Specimen Handling:
1. Wait for a clot to fully form (serum samples only).
2. Centrifuge the sample at 1300 to 2000 g (3000 to 3500 RPM) for 10 to 15 minutes.
3. All samples must be centrifuged within two hours of collection.
4. Tubes containing a gel separator can be stored at the appropriate temperature until they are ready to be shipped to the laboratory.
5. If there is no gel separator present, aliquot the serum/plasma into an appropriately labeled plastic screw cap transport vial. The original tube type must be written on the label of the aliquot tube (i.e., red top serum, EDTA plasma or Lavender top). The serum/plasma must be aliquoted within two hours of collection. Store the aliquoted sample at the appropriate temperature until it is ready to be shipped to the laboratory.

Sample Volume:
• Preferred: 1 mL
• Minimum: 0.5 mL

Specimen Stability:
• Refrigerated (preferred): 7 days
• Frozen: 3 months
• Ambient: 4 days

Rejection Criteria:
• Gross hemolysis
• Gross icterus
• Gross lipemia
• Unlabeled or improperly identifiable specimens
• Samples that have not been handled or transported to the laboratory at the proper temperature
• Samples that have exceeded the stated stability
• Wrong tube type/anticoagulant
• Uncentrifuged or improperly centrifuged samples
• QNS

Specimen Transportation:
1. Tubes containing gel must be centrifuged within 2 hours of collection.
2. Red-top tubes should be centrifuged, and the serum aliquoted into a plastic vial within 2 hours of collection.
3. Store specimens at 2 to 8° C until they are ready to be shipped.

Reference Interval(s):
AGE GENDER LOWER LIMIT UPPER LIMIT
< 1 year All 20 U/L 67 U/L
< 7 years All 21 U/L 44 U/L
< 12 years All 18 U/L 36 U/L
< 19 years Male 14 U/L 35 U/L
< 19 years Female 13 U/L 26 U/L
≥ 19 years Male 0 U/L 40 U/L
≥ 19 years Female 0 U/L 32 U/L

LOINC Codes: 30239-8

CPT: 84450

Test Principle: This assay follows the recommendations of the IFCC but was optimized for performance and stability. AST catalyzes the transfer of an amino group between L aspartate and 2 oxoglutarate to form oxaloacetate and L glutamate. The oxaloacetate then reacts with NADH, in the presence of malate dehydrogenase (MDH), to form L malate and NAD+. Pyridoxal phosphate serves as a coenzyme in the amino transfer reaction. It ensures full enzyme activation. The rate of the NADH oxidation is directly proportional to the catalytic AST activity. It is determined by measuring the decrease in absorbance. Assay performed using a Roche Cobas c503 test platform. AST measurements, performed with this device, in human serum and plasma are used as an aid in diagnosis of hepatocellular injury and in monitoring chronic liver injury.

Specimen Retention: 7 days

For diagnostic purposes, the results should always be assessed in conjunction with the patient’s medical history, clinical examination and other findings.