What tests are suggested for the diagnosis of liver disease?
The following screening tests are usually recommended when liver disease is suspected in a pet: a complete blood count (CBC), a serum biochemical profile, and a urinalysis.
What might these tests indicate if my pet has liver disease?
These screening tests may support the presence of liver disease or a reduction in the functional capacity of the liver, but they are not necessarily specific for liver disease.
Complete Blood Count (CBC)
The complete blood count (CBC) involves the evaluation of the red blood cells, the white blood cells, and the platelet components of a blood sample. A hematology (blood) analyzer will provide your veterinarian with the total numbers of these cells, and evaluation of a blood smear will allow your veterinarian to look at the physical characteristics of these cells.
One of the most dramatic features of liver disease is icterus, which is the accumulation of a yellow pigment in the blood and tissues called bilirubin. Icterus is recognized as jaundice, which is seen as a yellow discoloration of the skin, mucous membranes, or sclera (whites of the eyes). In addition to being a feature of liver disease, icterus can occur due to the destruction of red blood cells (a condition known as hemolysis). Obviously, it is important to differentiate between these two causes of icterus.
"One of the most dramatic features of liver disease is icterus, which is the accumulation of a yellow pigment in the blood and tissues called bilirubin."
The CBC will also allow your veterinarian to determine whether red blood cell numbers are within the normal range. This can be done by an actual count of the total number of red blood cells, or by determination of the hematocrit or the packed cell volume. These latter two measurements determine the proportion of the blood that is occupied by the red blood cells. If these parameters are within the normal range, then hemolysis is unlikely. An elevated reticulocyte count provides more evidence of hemolysis. Reticulocytes are immature red blood cells that are produced by bone marrow. If hemolysis is present and the red blood cells are being destroyed, the natural response of body is to release reticulocytes from the bone marrow.
Liver disease may occasionally result in changes in the shape of red blood cells. These shape changes include increased numbers of target cells. Normal red blood cells are round and uniformly red. In dogs, they also have a small, central, pale area. Target cells are red blood cells that look like a target, with alternating pale and red rings. This feature is thought to occur because of a change in the red blood cell membrane structure caused by liver disease.
Occasionally, increased numbers of small red blood cells (microcytes) or spiculated (spike-shaped) red blood cells (acanthocytes) may be found in association with liver disease.
The CBC may indicate the presence of inflammation or infection characterized by an increase in the number of white blood cells.
Serum Biochemistry Profile
The serum biochemistry profile requires a separate blood sample from which the serum (the liquid portion of blood) is separated from the cellular portion of blood. Serum contains many substances, including enzymes, proteins, lipids (fats), glucose (sugar), electrolytes, and metabolic waste products.
The biochemistry profile involves many tests that are helpful in the diagnosis of liver disease. Changes in the biochemistry profile do not tell us exactly what type of liver disease is present, but may be supportive of liver disease and may suggest certain categories of liver disease.
The most useful enzymes to measure with suspected liver disease include:
ALT (alanine aminotransferase). This enzyme is found within individual liver cells. When liver cells are damaged for any reason, an increased amount of the ALT enzyme is released into the bloodstream. This increase can be detected in a serum sample. Drugs, including corticosteroids and phenobarbital, may increase ALT values.
AST (aspartate transaminase). This enzyme is also found within liver cells and is released into the serum when the liver is damaged. Steroid-containing drugs do not result in significant increases in this enzyme. Since other body tissues also contain AST, increases in this enzyme are not liver-specific.
ALP (alkaline phosphatase). This group of enzymes is found on the membranes (outer lining) of liver cells and cells of the bile ducts. Bile ducts carry bile from the liver to the gall bladder and then into the intestine. Liver conditions that result in obstruction of bile flow (cholestasis) may result in increased serum ALP concentrations. Other causes of increased serum ALP concentrations include certain drug therapies (especially steroids and anticonvulsants) and some conditions related to skeletal bones. In young, rapidly growing dogs, it is normal to see elevations in serum ALP. It is not normal to see elevations in serum ALP in growing kittens.
GGT (gamma-glutamyltransferase). GGT is an enzyme associated with the membranes of cells from the bile ducts, as well as the membranes of cells from the pancreas and kidneys. Increased GGT production, and therefore increased serum concentrations of GGT, are found in liver conditions that result in cholestasis (impaired bile flow), especially those conditions resulting in increased growth or proliferation of bile ducts. Steroid-containing drugs may also increase serum GGT concentrations.
Total Bilirubin. In healthy animals, red blood cells are normally removed from circulation when they become aged. These red blood cells are broken down into several components, one of which is bilirubin. Bilirubin is processed in the liver and subsequently released from the liver into the intestines as a component of bile.
- Hyperbilirubinemia (an increased concentration of bilirubin in blood) can occur from increased destruction of red blood cells (hemolysis) or because of some problem with the uptake, processing, or release of bilirubin by the liver. The accumulation of bilirubin in tissues causes the yellow color or jaundice that can be seen with these conditions. As discussed in the CBC section, it is important to analyze the red blood cell parameters to rule out increased red blood cell destruction as a cause of hyperbilirubinemia.
- Liver diseases that can result in increased bilirubin concentrations in the serum can be divided into problems with the uptake of bilirubin, problems with the internal processing of bilirubin, and problems with the excretion of bilirubin into the bile. Sometimes, by measuring the two different components of total bilirubin (unconjugated and conjugated bilirubin), we can try to determine the location of the problem.
- Other liver enzyme changes may give your veterinarian further clues in the interpretation of bilirubin changes. For example, if the ALT enzyme is markedly increased, it suggests that damage within the liver is contributing to the problem with bilirubin processing. If the GGT enzyme is elevated, this would suggest that there is a problem with excretion of bilirubin from the liver into the bile (cholestasis).
Serum albumin, glucose, urea, and cholesterol determinations. The liver manufactures thousands of substances. Glucose, urea, cholesterol, and albumin are among those substances made by the liver that are routinely measured in a serum biochemistry profile. If there is a severe decrease in liver function, this may be reflected by a decrease in one or more of these substances.
Urinalysis
The kidneys constantly filter the blood to remove toxins and other wastes and concentrate these extracted materials in the urine. Therefore, there may occasionally be changes in the urinalysis before significant changes are noted in blood testing.
An increase in the amount of bilirubin in a urine sample may indicate liver disease that is not yet appreciable on a serum biochemistry profile. The presence of specific crystals in urine, such as bilirubin crystals or ammonium biurate crystals, may also point to underlying liver disease.
Are serum biochemistry tests always abnormal with liver disease?
No, unfortunately not. Occasionally, we may suspect liver disease because of the clinical signs that your pet is exhibiting, but any changes on the screening test may be subtle or non-existent.
What other tests are available for the diagnosis of liver disease?
Serum bile acid concentrations are used to evaluate the functional capacity of your pet's liver.
The liver makes bile acids using cholesterol. Bile acids are a major component of bile, which is stored in the gall bladder. Following a meal, the gall bladder contracts, releasing bile into the intestine. After assisting in the processes of digestion, most of the bile acids are re-absorbed into the blood from the intestines and are subsequently removed from the circulation by the liver.
This normal circulation of bile acids forms the basis for the serum bile acid test. After a 12-hour fast, a blood sample is taken. Your pet is immediately fed a small meal that should stimulate the gall bladder to contract. A second blood sample is taken two hours after the meal. The serum bile acid concentration is measured in both samples and the results are compared. If the liver is functioning normally, the amount of bile acids in both the fasted and post-feeding serum sample will be low, but slightly higher after the meal.
If liver function is reduced for any reason, the concentration of bile acids in circulation may be increased either in the post-feeding sample or possibly in both samples. Occasionally, the difference between the fasting and post-feeding samples may suggest specific disease processes. Increases, especially in the post-feeding samples, usually only provide further evidence of liver disease. See handout “Bile Acid Test” for more information.
Are there any tests that will tell me exactly what kind of liver disease my pet has?
Occasionally, the combination of these screening tests and a thorough clinical history and physical examination will suggest a likely cause of the liver disease. However, in most instances, further testing is required. This testing may include ultrasound examination of the liver and gall bladder.
During this examination, the ultrasonographer may take samples of any free fluid that is detected in the abdominal cavity and may take a small biopsy sample or a fine needle aspiration of the liver and/or gall bladder if abnormalities are observed.
Since the liver produces the proteins involved in blood clotting, it is necessary to determine the amount and functional activity of these proteins before proceeding with any biopsies. Other imaging modalities may also be considered, including a CT scan, an MRI, or a contrast study with radiopaque dye injected into the blood vessels.
"Since the liver produces the proteins involved in blood clotting, it is necessary to determine the amount and functional activity of these proteins before proceeding with any biopsies."
A sample of aspirated material from the gall bladder or liver may be submitted for culture if it is suspected that an infection is causing the liver disease. If there is reason to suspect leptospirosis, a blood sample may be submitted to determine a leptospirosis titer (see handout “Antibody Titers” for further information).
A larger biopsy sample may be obtained via laparoscopic surgery. Full abdominal exploratory surgery can also be considered if laparoscopic surgery is not an option. In addition to looking at the biopsy samples under the microscope, the copper level in the tissue can also be assessed. Certain types of liver disease, especially some breed-related conditions, cause the liver to store excess copper.
If these other tests are needed to determine what kind of liver disease is present, why bother with the screening tests at all?
The screening tests allow your veterinarian to document whether your pet has liver disease. Signs of liver disease include poor appetite, vomiting, lethargy, increased drinking and urination, and yellowing of the skin. These clinical signs can be found in other diseases, so it is important to rule out other conditions. These screening tests are minimally invasive and much lower cost than the biopsies and ultrasound evaluations discussed above.
In addition, these tests allow your veterinarian to monitor how your pet is responding to any treatments that are undertaken. Normalization of liver enzymes is a good indicator of response to treatment.
