84450
Aspartate Aminotransferase (AST) (SGOT) Blood Test
CPT code 84450 refers to the laboratory procedure for measuring the level of aspartate aminotransferase (AST), formerly known as serum glutamic-oxalacetic transaminase (SGOT), in a patient's blood. AST is an enzyme primarily found in the liver, heart, skeletal muscles, and kidneys. When these tissues are damaged or diseased, AST is released into the bloodstream, making it a vital biomarker for assessing tissue integrity. In clinical practice, AST is most frequently measured to evaluate liver health. While it is less specific to the liver than alanine aminotransferase (ALT)—because AST is also found in significant amounts in the myocardium and skeletal muscles—the ratio of AST to ALT (the De Ritis ratio) provides critical diagnostic clues for differentiating various types of liver injury. For instance, an AST/ALT ratio greater than 2:1 is often suggestive of alcoholic liver disease. Beyond hepatology, AST levels are historically significant in the context of cardiac care, as levels rise within 6 to 10 hours following an acute myocardial infarction and peak within 24 to 48 hours, though modern troponin assays have largely supplanted AST for this specific indication. The laboratory process involves a venipuncture to collect a serum or plasma sample, which is then analyzed using kinetic spectrophotometric methods. In this assay, AST catalyzes the transfer of an amino group from aspartate to alpha-ketoglutarate, forming oxaloacetate and glutamate. The oxaloacetate is then reduced to malate by malate dehydrogenase, a reaction that consumes NADH. The rate of decrease in NADH absorbance, measured at 340 nm, is directly proportional to the AST activity in the sample. Results are typically reported in units per liter (U/L). Accurate interpretation requires consideration of the patient's age, gender, and concurrent medications, as certain drugs like statins, antibiotics, and anticoagulants can artificially elevate levels.
Clinical Indications
- Evaluation of suspected liver disease or hepatitis
- Monitoring of patients taking potentially hepatotoxic medications such as statins or methotrexate
- Differential diagnosis of jaundice or abdominal pain
- Assessment of liver damage in cases of chronic alcohol abuse
- Screening for liver injury following exposure to environmental toxins
- Evaluation of muscle injury or rhabdomyolysis
- Historical monitoring of myocardial infarction when other markers are unavailable
Procedure Steps
- Verification of patient identity and review of the laboratory requisition
- Preparation of the venipuncture site using aseptic technique
- Collection of whole blood via venipuncture into a serum separator tube (SST) or lithium heparin tube
- Allowing the blood to clot for 30 minutes if an SST is used
- Centrifugation of the specimen to separate the serum or plasma from cellular components
- Inspection of the sample for hemolysis, as lysed red blood cells can falsely elevate AST levels
- Loading the sample into an automated clinical chemistry analyzer
- Performance of a kinetic enzymatic assay involving the conversion of aspartate and alpha-ketoglutarate
- Calculation of enzyme activity based on the rate of NADH oxidation observed via spectrophotometry
- Validation of results against quality control standards and reporting into the electronic health record
Coding Guidelines
- CPT 84450 should not be reported separately if performed as part of a Hepatic Function Panel (CPT 80076)
- CPT 84450 should not be reported separately if performed as part of a Comprehensive Metabolic Panel (CPT 80053)
- If the test is repeated on the same day for a clinical reason, modifier 91 (Repeat clinical diagnostic laboratory test) may be appropriate
- This code represents a quantitative assay; for qualitative screens (rare for this analyte), alternate codes would be used
- Ensure that the documentation supports medical necessity, such as monitoring for drug toxicity or evaluating specific symptoms like jaundice