Alan H. Rebar, DVM, PhD, DACVP
The liver performs a wide variety of different and seemingly unrelated functions. For example, it plays a central role in plasma protein synthesis, carbohydrate metabolism, lipid metabolism, and detoxification of both endogenous and exogenous substances. In addition, the liver is the site of bilirubin metabolism and bile synthesis, as well as synthesis of most circulating coagulation factors. The Kupffer cells of the hepatic sinusoids form one of the major elements of the monocyte-macrophage continuum (mononuclear phagocyte system).
The diversity of hepatic function suggests that a chemistry organ panel assessing the liver must also be diverse. The screening panel includes tests of primary importance as well as a group of additional tests to be more closely evaluated when abnormalities are present in any of the screens.
Primary Hepatic Panel
Serum Alanine Aminotransferase (ALT)
Serum alanine aminotransferase (ALT) is probably the most accurate indicator of liver disease in small animal medicine. However, it is important to realize that ALT is not a liver function test but rather an indicator of hepatocyte injury. ALT is a liver-specific enzyme present in high concentrations within the cytoplasm of hepatic parenchymal cells. As such, serum ALT activity is obviously increased with necrosis. However, a common response to non-lethal hepatocellular injury involves membrane blebbing with subsequent release of cytoplasmic-rich vesicles such that increased ALT activity is seen in the serum. Therefore, in a general way, the degree of elevation correlates not with the severity of hepatocellular damage but rather with the number of hepatocytes involved. In other words, diffuse fatty change may result in more extreme ALT activity elevations than focal hepatic necrosis.
As with other serum enzymes, interpretation of ALT values is largely dependent upon circulation dynamics. Serum alanine aminotransferase activity reaches maximal elevation approximately 48 hours after acute injury. The half-life of ALT is approximately 2 to 4 days in the dog and approximately 6 hours in the cat. Consequently, elevations of ALT activity following single episodes of hepatocellular damage will be transient; continuous and persistent elevations imply ongoing hepatocellular damage.
Serum Alkaline Phosphatase (ALP)
Serum alkaline phosphatase (ALP) is a membrane-bound enzyme produced at the bile canalicular surface of hepatocytes. Increased ALP production is induced whenever cholestasis occurs with resultant elevation in circulating enzyme activities. Thus, ALP is not an indicator of hepatocellular leakage, as is ALT; instead ALP is used as an indicator of either intrahepatic or extrahepatic biliary obstruction.
Unfortunately, ALP is not liver-specific; the enzyme is also found in bone, placenta, intestine, kidney and leukocytes. In addition, both exogenous steroid administration and endogenous adrenal glucocorticoid production can induce the production of a second isozyme of ALP in the dog (but not in the cat). Furthermore, drugs such as primidone and phenobarbital can directly induce ALP production. In general, in dogs 2- to 3-fold elevations of ALP activity are regarded as non-specific and may be the result of liver disease, bone disease, or drug/exogenous steroid administration. Also in dogs, 4-fold elevations or greater are virtually always the result of cholestasis or induction of the corticosteroid isozyme of alkaline phosphatase.
Interpretation of serum ALP activity in cats is quite different. First, normal ALP activity in the liver of cats is much lower than in dogs. In addition, the circulating half-life of ALP in cats is significantly shorter than that of dogs. As a consequence, any elevation in ALP activity in cats is regarded as suggestive of cholestasis.
ALP elevations secondary to cholestasis may occur with or without concurrent elevations of ALT. Many acute conditions causing hepatocellular injury and ALT release also cause hepatocellular swelling and intrahepatic cholestasis. In contrast, many more chronic hepatic disorders are characterized by periportal fibrosis with resultant cholestasis and elevated ALP levels but little active hepatocellular degeneration.
Serum Gamma Glutamyl Transferase (GGT)
Serum gamma glutamyl transferase (GGT) is a second membrane bound enzyme associated with bile duct epithelium commonly included in the primary hepatic diagnostic panel. Both ALP and GGT are indicators of cholestasis. It has been suggested that GGT may be more useful than ALP because GGT activity elevations are not directly induced to a significant magnitude by glucocorticoids and drugs such as primidone. However, in most cases this distinction is academic; most drugs which directly induce ALP also cause hepatocellular swelling which secondarily causes intrahepatic cholestasis and elevated GGT activity.
Measuring both GGT and ALP activities is probably most useful in cats where elevations in ALP are often more subtle. Elevations of both enzymes simultaneously provides supportive evidence that cholestasis is present. In cats, a relatively greater increase in ALP than GGT is suggestive of hepatic lipidosis.
Total Protein (TP) and Albumin
The majority of the plasma proteins are produced in the liver and severe liver disease may be a cause of hypoproteinemia due to decreased production. Due to the relatively long half-lives of plasma proteins (7-10 days), such alterations are usually seen only in chronic liver disease. Hypoproteinemia of this type is usually predominantly the result of hypoalbuminemia.
If only total protein (TP) is measured (and not albumin), the hypoproteinemia of liver disease may be missed. This is because hepatic disease is sometimes accompanied by hypergammaglobulinemia (gamma globulins are produced by cells of the immune system rather than hepatocytes), which may keep TP levels in the normal range. Hypergammaglobulinemia can develop in chronic liver disease because there are increased levels of circulating foreign proteins which have not been removed by the liver; this results in systemic antigenic stimulation.
Secondary Hepatic Panel
Blood Urea Nitrogen (BUN)
In the liver, ammonia is metabolized to urea, the principal nitrogenous waste product of mammalian systems. The blood carries urea to the kidneys, where it is excreted as a part of the glomerular filtrate. In cases of reduced hepatic blood flow (congenital or acquired portosystemic shunts) and possibly with reduced functional hepatic mass, urea production from ammonia may be markedly reduced with a resultant decrease in circulating blood urea nitrogen (BUN) levels. It should be emphasized that a decreased BUN is not specific for liver disease of this nature; on the contrary, a common cause of decreased BUN is diuresis. Establishing liver disease as a cause of decreased BUN is best accomplished by demonstrating a concomitant elevation in circulating ammonia, by measuring pre and postprandial ammonia levels, or by measuring pre and postprandial bile acid levels. Both serum ammonia and serum bile acids are special tests not usually included in the large chemistry profile and therefore beyond the scope of this text.
Serum Bilirubin, Urine Bilirubin
When senescent RBCs are phagocytized and degraded by macrophages, the hemoglobin they contain is converted to heme and globin. The protein moiety, globin, is degraded to its amino acid constituents and recycled. The tetrapyrrole ring, heme, is enzymatically cleaved with release of iron and, following further degradation, is converted to free (unconjugated) bilirubin. Unconjugated bilirubin is complexed to albumin and circulated to the liver where it is conjugated with glucuronic acid and excreted in bile as bilirubin diglucuronide.
Sera from normal individuals contain a small amount of both conjugated and unconjugated bilirubin. Increases in total circulating bilirubin may result from prehepatic, intrahepatic or posthepatic causes. Prehepatic elevations are the result of hemolysis; increased breakdown of RBCs leads to increased levels of circulating bilirubin. As might be expected in the acute phase of hemolysis, the majority (more than 75%) of the elevations in bilirubin are usually the result of elevation in unconjugated (indirect) bilirubin. Elevations due to intrahepatic cholestasis are usually the result of increases in both conjugated (direct) and unconjugated bilirubin. Elevations resulting from posthepatic cholestasis usually feature predominant (75%) elevations in conjugated bilirubin acutely, although levels of unconjugated bilirubin may also be increased. However, the reader is cautioned that these patterns of elevation are suggested as general guidelines and become increasingly less reliable as disease processes progress.
Circulating conjugated bilirubin passes the glomerulus with the glomerular filtrate and is excreted in the urine. Therefore elevated urine bilirubin levels may also be used as an indicator of hepatic disease with cholestasis particularly in the dog, which normally has a low normal renal threshold for bilirubin. In the dog, normal urine contains only small amounts of bilirubin when evaluated by standard reagent dip strip methods; an increased amount is therefore a significant finding. However, occasional increased urine bilirubin with no evidence of liver disease is seen in some dogs. The cause of this phenomenon is uncertain. In other cases, bilirubinuria may precede bilirubinemia in the progression of liver disease. Since only conjugated bilirubin passes the glomerulus, urine bilirubin levels do not usually reflect presence of prehepatic bilirubinemia.
The normal cat has a high renal threshold for bilirubin; the reagent dip strip test is almost always negative even when serum bilirubin levels are significantly elevated. Positive urine bilirubin tests in cats are only obtained in the most severe cases of liver disease, usually after clinical icterus is apparent.
Urine bilirubin and serum bilirubin are included only as secondary liver screening tests because they are less sensitive indicators of cholestasis than ALP or GGT. As a general rule in dogs, ALP and GGT elevate earlier than urine bilirubin levels, which in turn can be detected earlier than elevations in serum bilirubin levels.
Delta bilirubin is conjugated bilirubin that has been bound to albumin. In previously used diazo reagent methods, all conjugated bilirubin, whether protein-bound (delta) or not, was measured as direct or conjugated bilirubin. Some newer assays are specific for non-protein-bound conjugated bilirubin, the fraction that most closely parallels active cholestasis. The delta bilirubin fraction is then calculated by subtracting unconjugated and conjugated bilirubin values from total bilirubin.
Delta bilirubin is not readily excreted and therefore has nearly the same circulating half-life as albumin. In contrast, both conjugated and unconjugated bilirubin are readily cleared. Consequently, when liver disease resolves, delta bilirubin persists while conjugated and unconjugated fractions are rapidly excreted. In people with liver disease, if total bilirubin is elevated and the major form is delta bilirubin, the prognosis is favorable. Although less is known with regard to animals, there is some evidence to suggest that the same is true in dogs.
Cholesterol and Triglycerides
Because the liver is central to lipid metabolism, hepatic disease greatly influences circulating lipid levels. It is well established that serum cholesterol and triglycerides are often elevated in liver diseases in both man and animals. However, these tests are listed only as components of the secondary liver screen because they are far from specific for hepatic disease. Elevations occur in a large number of diseases such as pancreatitis, diabetes mellitus, hypothyroidism, etc. These two tests are therefore considered a part of several organ system panels. In contrast, very few conditions result in decreased serum cholesterol. The primary differential for hypocholesterolemia is reduced synthesis secondary to hepatic insufficiency.
Chronic severe liver disease can cause hypoglycemia or hyperglycemia. This is a reflection of reduced glycogen storage capacity and reduced functional hepatic mass. The presence of hypoglycemia in cases of obvious liver disease is therefore a poor prognostic sign. Hyperglycemia in liver disease is a postprandial event also due to reduced functional hepatic mass where there is no longer a place for glucose storage.
Reprinted with permission from Biochemical Profiling in the Dog and Cat, A.H. Rebar, G.D. Boon, J.A. Christian, The Gloyd Group, Inc., Ralston Purina Company, 1999.