The biochemical parameters used to assess liver pathology may be divided into two classes: the enzymes that reflect liver damage and/or cholestasis (discussed below) and the indicators of liver function (bile acids, ammonia, albumin etc.). Serum enzyme screening for hepatobiliary disease is common in veterinary practice. Clinical signs associated with liver disease are wide-ranging and often non-specific, and consequently laboratory profiles are often run in patients that have a constellation of clinical signs that includes one or more of those seen in liver disease. More frequently, patients that are just "not well", or those that are clinically normal (in for a "check up" often young patients coming in for vaccination/deworming or geriatric patients), or because surgery is being contemplated, have test "panels" conducted that almost invariably include liver enzymes. It is against this background that one should assess the usefulness of laboratory tests. It is just too simplistic to evaluate liver enzymes by comparing the serum activity in patients with liver disease with those that are healthy. For a test to be really useful, it must be abnormal in most, patients with liver disease most of the time and normal in most sick patients, with signs that could represent liver disease as well as the latter "healthy" groups, most of the time.

As patients may be presented some time after the damage first occurred, a one-off serum activity is difficult to interpret in terms of severity. Furthermore, pathophysiological processes in diseases that are not primarily hepatic may generate fairly substantial serum activity (secondary liver disease or induced enzyme activity). Even those primary diseases, such as parenchymal damage, cholangitis, cholangiohepatitis, chronic hepatitis and diffuse neoplasia, may be accompanied by negligible or no increases in serum enzyme activity.

This paper compares the diagnostic utility of liver enzymes, when assessed against normal animals with that derived from assessment in a "routine" clinical environment. Insights gained during a long-term toxicology trial are also presented. For the purpose of this paper, liver disease will be broadly categorised into Acute hepatopathy (AH), Chronic hepatopathy, including both chronic fibrotic as well as vascular diseases (CH), Biliary pathology (BP) and Endocrine hepatopathy (EH).

Alanine Aminotransferase (Transaminase) (ALT)

ALT is accepted as being essentially liver specific in the dog. Peak serum activity is proportional to the number of hepatocytes affected, but gives no indication as to whether the disease is diffuse or focal, and does not reflect the severity of the disease or its reversibility. The reported plasma half life is of the order of two-and-a-half days, implying that, after serum peak levels, a halving every 2 to 3 days is a good prognostic indicator. Earliest sensitivity data (Center's 1985, 150-case report) range from 47% for CH, 77% in BP, 40% in EH and 100% in AH. Our own data (165 cases of suspected liver disease) were 45%, 60%, 67% and 71% respectively.

Data obtained from our hospital database (1252 cases on which serum ALT was determined), the data were 47%, 100%, 72% and 75% respectively. The interesting, if somewhat disappointing, index derived from this survey, is that the Predictive value of a positive result (ALT > 40 U/l) for the presence of liver disease is only 18% (i.e., if one obtains a laboratory result of ALT > 40 U/l (our top normal), then the probability that the patient has hepatic disease is 18%). If one sets the cut-off at twice-top-normal, however, this improves to 29%.

Alkaline Phosphatase (ALP)

The presence of several isoenzymes and a unique sensitivity to drug and cholestatic induction by de novo synthesis, solubilization from membranes by bile salts and/or elution from membranes, is reported to make ALP a test of relatively low specificity. However, its clinical utility arises from its reported sensitivity in detecting hepatobiliary disease. Interpretation of results in "sick" dogs, that do not have canine Cushing's disease, is often confounded by the difficult-to-predict effect of endogenously produced and/or iatrogenically administered corticosteroids. Plasma half life is reported to be about 72 hrs in dogs. Early published sensitivity data (Center, 1985) range from 44% for CH, 91% in BP, 90% in EH and 57% in AH. Our own data (165 cases) were 49%, 67%, 67% and 74% respectively.

Data obtained from our hospital database (1277 cases on which serum ALP was determined), the data were 50%, 100%, 81% and 71% respectively. The predictive index, derived from this survey, is that the Predictive value of a positive result (ALP > 190 U/l (our top-normal) for the presence of liver disease is only 21%. If one sets the cut-off at twice-top-normal, however, this improves to 32%. So, surprisingly, our data suggest that ALP is mare specific than is generally believed (when compared with ALT).

Gamma-Glutamyl Transpeptidase (GGT)

Although present in many tissues, the majority of serum GGT is derived from the liver. Although abundant in renal tubular epithelium, it is accepted that serum levels are not elevated after kidney damage as the released enzyme is lost in the renal filtrate. Activity usually parallels that of ALP, but it is perhaps less influenced by hepatocyte necrosis and more by biliary epithelium disease. It appears that serum GGT is more specific (87%) than serum ALP (51%) in the detection of hepatobiliary disease, but less sensitive (50%) than ALP (80%) (Center, 1992). The measurement/interpretation of ALP and GGT in series improves the specificity markedly from 46% to 91%. In an aflatoxicosis toxicology trial conducted on a small number of dogs, GGT was shown to be very insensitive, only exceeding upper-normal on 34 of the 74 dog-days (11 of which by only one unit) in animals fed a toxic dose (representing a 46% sensitivity).

Glutamate Dehydrogenase (GLD)

GLD is a mitochondrial enzyme, and therefore requires fairly substantial cell damage before it is released Increased GLD is reported to be a fairly specific indicator of liver damage. Although abundant in renal tubular epithelium, it is accepted that serum levels are not elevated after kidney damage as the released enzyme is lost in the renal filtrate. Sensitivity of this enzyme for hepatic disease is reported to be high at 92%, and it is believed to reflect necrosis of hepatocytes. In the same aflatoxicosis toxicology trial, GLD was shown to be more sensitive than GGT, exceeding upper-normal on 45 of the 74 dog-days (most elevations being substantial) in animals fed a toxic dose (representing a 61% sensitivity).

Aspartate Aminotransferase (Transaminase) (AST)

Two AST isoenzymes are found, one cytosolic and one mitochondrial. The half life is reported to be about 12 hours in dogs. This enzyme is released into the blood with increased cell membrane permeability and cellular necrosis (when the mitochondrial isoenzyme is released). Like ALT, the magnitude of increase said to be proportional to the number of hepatocytes injured but does not indicate the functional status of the organ. It is reported to be a good indicator of degree of necrosis as well as a good screening test with a sensitivity of 88%. Since AST is found in many organs, including muscle, heart and liver it is not a specific indicator of hepatic damage and if elevations occur in the absence of elevations of ALT, measurement of serum Creatine kinase, a muscle specific enzyme, may be required to rule out muscle damage. Since AST does not appear to have an advantage over ALT as a marker of hepatocyte damage in terms of liver specificity, its value as an additional test should be questioned. It is not routinely used to assess hepatic injury in dogs as it does not appear to have an advantage over ALT as a marker of hepatocyte damage. However, AST was increased in 2/3 of 14 dogs with hepatic abscesses. In the late stages of hepatic lipidosis when the lipid accumulation becomes excessive, AST is reported to elevate. Increased AST is reported to be a sensitive indicator of metastatic (secondary) liver disease in dogs. The author's institution does not, generally, make use of this enzyme in dogs, preferring to rely on ALT and ALP.

Until time-and-while a novel serum, liver-specific enzyme is proposed, the use of ALT and ALP (with the possible occasional use of GDH), probably supplies the average veterinary clinician with sufficient information about hepatocyte integrity, cholestasis and steroid induction provided that the limited sensitivity and poor predictive value are borne in mind.