Severe tissue damage results in more of the mitochondrial enzyme being released. Elevated AST levels are found in hepatopathies, muscular dystrophy, and damage to internal organs. Increased levels of AST however are generally a result of liver disease associated with some degree of hepatic necrosis such as cirrhosis, carcinoma, viral or toxic hepatitis, and obstructive jaundice. Following a myocardial infarction, serum levels of AST are elevated and reach a peak 48 to 60 hours after onset.
Vitro AST reagent is based on the recommendation of the IFCC5. The series of reactions involved in the assay system are as follows:
- The amino group is enzymatically transferred by AST present in the specimen from aspartate to the carbon atom of 2-oxoglutarate yielding oxaloacetate and L-glutamate.
- Oxaloacetate is reduced to malate by MDH present in the reagent with the simultaneous oxidation of NADH to NAD.
- The rate of oxidation of the coenzyme NADH is proportional to the AST activity in the specimen. It is determined by measuring the decrease in absorbance at 334 / 340 / 365 nm correspondingly. Lactate dehydrogenase is included in the reagent to convert endogenous pyruvate in the specimen to lactate during the lag phase prior to measurement.
AST and ALT are found in many extra-hepatic tissues including heart, skeletal muscle, erythrocytes, lung, brain and kidney, although the extra-hepatic contribution in plasma is less for ALT making it more liver specific. Muscle damage, acute cardiac failure and shock may rarely cause an increase in AST as high as 10 times the upper limit of the reference range. Elevation to six times normal has been described in hypothyroidism, but this is of muscular rather than liver origin and is accompanied by a rise in creatine kinase activity.
