What Is Aspartate Aminotransferase (AST) Test?

Aspartate Aminotransferase (AST) test

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The AST test is a crucial tool in the assessment of liver function and the diagnosis of liver damage. Despite its limitations in specificity, it provides valuable information when used alongside other tests such as ALT. Advances in detection techniques and standardization of reference intervals have improved the accuracy and reliability of AST measurements, making it an indispensable component of clinical diagnostics.

Aspartate aminotransferase (AST), also known as serum glutamic-oxaloacetic transaminase (SGOT), is an enzyme found in various tissues of the body, including the liver, heart, muscles, and kidneys. The AST test measures the amount of this enzyme in the blood and is commonly used to assess liver health and diagnose liver damage. This article provides an overview of the AST test, its clinical significance, and the methods used for its detection.

Clinical Significance of AST

AST is an important biomarker for liver function. Elevated levels of AST in the blood can indicate liver damage or disease, as well as damage to other tissues where the enzyme is present. The AST test is often used in conjunction with the alanine aminotransferase (ALT) test to provide a more comprehensive assessment of liver health. While ALT is more specific to the liver, AST can be elevated in conditions affecting other organs, making it a less specific but still valuable diagnostic tool1 2.

Detection Techniques

Several techniques are used to measure AST levels in the blood, each with its own advantages and limitations. These methods include:

• Colorimetric and Spectrophotometric Methods: These are traditional techniques that measure the change in color or absorbance of a solution as a result of the enzymatic reaction involving AST. They are widely used due to their simplicity and cost-effectiveness1.
• Chemiluminescence and Fluorescence: These methods offer higher sensitivity and specificity by detecting light emitted from a chemical reaction or fluorescence from a fluorophore, respectively1.
• Chromatography and UV Absorbance: These techniques separate and quantify AST based on its interaction with a stationary phase and its absorbance of UV light1.
• Radiochemical and Electrochemical Methods: These methods involve the use of radioactive isotopes or electrochemical sensors to detect AST activity, providing high sensitivity and accuracy1.

Reference Intervals and Biological Variation

The reference intervals for AST can vary based on factors such as age, gender, and geographic location. A multicenter study standardized the reference intervals for AST, ALT, and γ-glutamyl transferase (GGT) across different regions, finding that while there were some statistical differences, they were not clinically significant. The study established a common reference interval for AST of 11-34 U/L2. Additionally, biological variation in AST levels can be influenced by individual factors, and published data show a wide range of within-subject variation3.

Diagnostic Utility and Limitations

While AST is a valuable marker for liver health, its diagnostic utility is limited by its lack of specificity. Elevated AST levels can result from conditions affecting other tissues, such as the heart or muscles. Studies have suggested limiting AST testing to cases where ALT levels exceed a certain threshold to reduce unnecessary testing and healthcare costs4 5. However, AST remains an important predictor of mortality, particularly for liver-related conditions, and elevated AST levels have been associated with a significant reduction in life expectancy9.