The enzyme primarily measured to assess hepatocellular injury in patients suspected of liver damage or viral hepatitis is Alanine Aminotransferase (ALT).

- ALT is an enzyme found predominantly in the cytosol of hepatocytes, making it a highly specific marker for liver cell injury.
- When liver cells are damaged, ALT is released into the bloodstream, leading to elevated serum levels.

- While Aspartate Aminotransferase (AST) is also elevated in liver injury, it is less specific because it is found in other tissues such as the heart, muscle, and kidneys.

- Alkaline Phosphatase (ALP) and Gamma-glutamyl Transferase (GGT) are enzymes more indicative of cholestasis or biliary tract disease, rather than primary hepatocellular damage.
- ALP rises in conditions involving bile duct obstruction, and GGT is often elevated with alcohol-related liver disease and biliary pathology.

Therefore, ALT is the most reliable enzyme for detecting and monitoring hepatocellular injury, including viral hepatitis and other causes of liver cell damage.

Reference: Harrison’s Principles of Internal Medicine, 20th Edition, Volume 2, Chapter 304/ Page 2799