The Clinical Approach to Abnormal Liver Enzymes

A common clinical situation is finding abnormal liver enzymes on a biochemical profile. In some cases abnormal liver enzymes are first identified in the clinically normal patient on a routine health screen or as a preanesthetic screen prior to an elective surgery. Froe frequently abnormal liver enzymes are identified in sick patients as part of a complete diagnostic work-up. Some of those sick patients may have primary liver disease but most usually have some other primary disorder with a so-called secondary “reactive” liver changes. The clinician is often left with questions on how to approach these case two case scenarios in a logical and reasonable manner for both the patient and the client. In the following pages I describe how to interpret abnormal liver tests and how I clinically approach those cases. The identification of abnormal liver enzymes (ALT, AST, ALP, GGT) usually indicates liver damage but rarely provides a diagnosis or etiology. Abnormal liver enzymes are common and in a study of 1,022 blood samples taken from both healthy and sick dogs and cats in one diagnostic laboratory found 39% had ALP increases and 17% had ALT increases.1 The identification of liver biochemical abnormalities in conjunction with the clinical findings suggest certain diagnostic possibilities and will indicate further steps into the investigation of possible liver disease. Liver biochemical enzymes can be insensitive or nonspecific for primary liver disease and in addition some of the enzymes can have isoenzymes from other tissue not associated with the liver. An understanding of the liver biochemical tests is essential when evaluating the patient in question. Liver biochemical test abnormalities are categorized into groups that reflect 1) hepatocellular injury, 2) cholestasis or 3) tests of impaired metabolic function or synthetic capacity.