Clinical enzymology uses measurements of enzyme activity in blood and other body fluids to diagnose disease, monitor disease progression, and assess treatment response. Common diagnostic techniques include ELISA, Western blot, and SDS-PAGE. When cells are damaged, intracellular enzymes leak into the bloodstream, where their measurement provides valuable diagnostic information.
Principles of Diagnostic Enzymology
Enzymes detected in plasma originate either from plasma-specific functions, such as clotting factors, or from leakage from cells. Tissue damage releases cellular enzymes into the circulation, and the pattern of enzyme elevation reflects the affected organ. The magnitude and time course of enzyme elevation provide information about the extent and duration of injury. Isoenzymes, different molecular forms of the same enzyme, can provide even greater tissue specificity.
Cardiac Markers
Cardiac troponins are the preferred markers for myocardial infarction, but traditional enzyme markers remain clinically useful. Creatine kinase exists as three isoenzymes: CK-MM (skeletal muscle), CK-MB (cardiac muscle), and CK-BB (brain). CK-MB rises within 4 to 6 hours of a heart attack, peaks at 24 hours, and returns to normal within 72 hours. The ratio of CK-MB to total CK helps distinguish cardiac from skeletal muscle damage.
Lactate dehydrogenase has five isoenzymes formed from H and M subunits. LDH1 (H4) predominates in the heart, while LDH5 (M4) is more abundant in the liver and skeletal muscle. In myocardial infarction, LDH1 exceeds LDH2, producing a flipped pattern. LDH rises more slowly than CK-MB but remains elevated longer, making it useful for late-presenting patients.
Liver Enzymes
Liver enzyme tests help diagnose and monitor liver disease. Alanine aminotransferase is highly specific to the liver and is released from damaged hepatocytes. Marked elevation, often exceeding 10 times the upper reference limit, is typical of acute viral hepatitis or drug-induced liver injury. Aspartate aminotransferase is found in the liver, heart, skeletal muscle, and red blood cells, making it less specific. The AST-to-ALT ratio helps distinguish causes of liver disease. A ratio above 2 suggests alcoholic liver disease, while a ratio below 1 is typical of viral hepatitis.
Alkaline phosphatase is elevated in cholestatic liver disease, such as bile duct obstruction. Gamma-glutamyl transferase is also elevated in cholestasis and is particularly sensitive to alcohol-induced liver injury. GGT can confirm the hepatic origin of elevated alkaline phosphatase.
Pancreatic Enzymes
Amylase and lipase are the primary enzymes measured for diagnosis of pancreatitis. Serum amylase rises within 6 to 12 hours of onset and remains elevated for 3 to 5 days. However, amylase can also be elevated in other conditions including salivary gland disease, renal failure, and intestinal obstruction. Lipase is more specific for pancreatitis and remains elevated longer than amylase, making it the preferred diagnostic marker.
Bone Enzymes
Alkaline phosphatase is produced by osteoblasts and is elevated in conditions of increased bone turnover, including Paget disease of bone, healing fractures, and bone metastases. Bone-specific alkaline phosphatase provides greater specificity for bone disease compared to total alkaline phosphatase.
Enzyme Assay Methods
Clinical enzyme assays measure either catalytic activity or enzyme mass. Activity measurements use spectrophotometric methods to monitor the rate of substrate consumption or product formation, typically coupled to NADH production or consumption monitored at 340 nm. Modern assays are performed on automated analyzers that can process hundreds of samples per hour. Results are expressed as international units per liter, where one international unit catalyzes the conversion of one micromole of substrate per minute under specified conditions.
Preanalytical Considerations
Several factors affect enzyme measurements. Hemolysis releases intracellular enzymes from red blood cells, falsely elevating results for LDH and AST. Sample storage can affect enzyme stability, with some enzymes losing activity rapidly at room temperature. Circadian variation, exercise, and drugs can also influence enzyme levels. Reference intervals must be established for each enzyme in each laboratory, accounting for age, sex, and population differences.
