Last month we kicked off the idea of Analyte of the Month. With so many species to care for and so little time, we thought a brief review of the fundamentals of each analyte in our chemistry panels with species-specific highlights would be useful in gleaning as much as possible from each chemistry profile.
We decided to start with aspartate aminotransferase (AST) because it is included in all of our chemistry panels, and it can be a tricky analyte to interpret at times. Increases in AST activity can mean many things, and therefore it has to be interpreted in the context of the patient for a better diagnostic evaluation.
AST is not organ specific and can be found throughout the body in all species.1,2 Skeletal muscle contains the highest AST concentration, but it can be found in cardiac muscle as well. 1–5
The liver has the next highest concentration in the body followed by red blood cells and brain tissue.1,5 In reptiles, AST has even been identified in renal tissue.4 Regardless of the species, AST is essentially omnipresent.
Although this enzyme is found many places, its ability to act as an indicator of disease varies by species. Therefore, a careful diagnostic approach must be applied when attributing clinical significance or evaluating the meaning of changes in this analyte.
For example, in reptiles AST can be found in the kidneys, but ultimately acts as a poor indicator of renal function.4 Some species of turtles exhibit seasonal variations in this analyte.4 Meanwhile, in snakes it is found that, even in the face of liver disease, this analyte doesn’t change that much.4
AST elevations are commonly encountered in avian species. Due to the high sensitivity and low specificity for both hepatocellular leakage and skeletal muscle damage, challenges arise regarding interpretation.3–5 AST can quickly increase in response to intramuscular injections in birds, and recent IM treatments must be a consideration when elevations are noted.4
Increases in AST in rabbits correlate well with the degree of hepatocellular degeneration making this a more sensitive indicator of hepatic disease.4 In canine and feline patients, despite having high sensitivity for hepatobiliary disease, AST is significantly less specific. 2
In addition to AST being present throughout various organs of the body, there are also species-specific variations in half-life.2,5,6 This adds another factor to consider when using AST as a part of evaluating a patient’s health.
The known half-life of this analyte, in the animals reviewed, are depicted below:
Notice the wide variety of AST half-life between mammals. Whether veterinarians are considering infectious disease in dogs or muscle damage in cats, considering each animal's AST half-life is important for a more definitive diagnosis.
Consequently, interpreting AST in patients can be complicated. For example, a high level of AST could mean skeletal-muscle breakdown, hepatic disease, or hemolysis. So, with AST being essentially omnipresent, how should it be interpreted?
A rough approach to interpreting AST is together with CK (creatine kinase).3–5,7 This can help differentiate elevations due to skeletal muscle damage or hepatocellular leakage.
Since CK is a marker of skeletal muscle damage1–4,7,8, if elevated alongside AST, then skeletal muscle damage is more likely to be the cause of the elevated AST. In cases of chronic muscle damage, CK may be normal and AST may still remain elevated. 3
Although interpreting the AST value in context with other analyte data does not cover every possible case3, this approach can be loosely applied across all species as a start towards a more definitive diagnosis.
In cases where the specimen is hemolyzed, AST will be falsely increased. Special care should be taken during the venipuncture and specimen transport to avoid hemolysis, and in turn, erroneous results. The concentration of AST in red cells is roughly 15 times that of normal serum; therefore, hemolysis should be avoided9.
The test is run on a laboratory analyzer which uses an enzymatic rate method to measure the AST activity in serum/plasma. First an enzymatic reaction takes place. This is followed by a rate of change in light absorbance that is directly proportional to the AST activity in the sample.
Next month we will review CK and tie these analytes together in more detail. We’re also working on producing a pocket reference document with analyte information for each species, so stay tuned for that downloadable document coming soon.