Generally the following screening tests are recommended when liver disease is suspected: 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.
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.
One of the most dramatic features of liver disease is icterus (the accumulation of a yellow pigment in the blood and tissues). Clinically, icterus is recognized as jaundice. 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 try and differentiate between these two causes of icterus.
The CBC will allow us to determine whether or not 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, and by determination of the hematocrit or packed cell volume. These latter two measurements essentially determine the proportion of the blood that is occupied by the red blood cells. If these parameters are within the normal or reference range then hemolysis is unlikely.
Liver disease may occasionally result in changes in the shape of red blood cells. These shape changes include increased numbers of target cells. This means that instead of red blood cells that are round in shape and are uniformly red with a small central pale area (this central pallor is noted only in dogs), some red blood cells may look like a target with alternating pale and red rings. This feature is thought to occur because of a change in the red cell membrane structure as a result of 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.
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) and metabolic waste products.
The biochemistry profile contains many tests that are helpful in the diagnosis of liver disease. Although changes in the biochemistry profile may be supportive of liver disease, and these changes may suggest certain categories of liver disease, they will not tell us exactly what type of liver disease is present.
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 blood stream. This increase can be detected in a serum sample. Drugs, especially steroid-containing drugs, may also increase ALT values.
AST (aspartate transaminase)
This is another enzyme found within liver cells that will be 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)
These are a group of enzymes found on the membranes (outer lining) of liver cells and cells of the bile ducts (the bile ducts carry bile from the liver to the intestine). Liver conditions that result in obstruction of bile flow (termed 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.
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.
In healthy animals, red blood cells are normally removed from the circulation when they become aged. These red blood cells are broken down into a number of 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 two main sources: either increased destruction of red blood cells, 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 either of these conditions.
As discussed in the CBC section, it is important to analyze the red blood cell parameters in order 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, with the internal processing of bilirubin, or with the excretion of bilirubin into the bile.
Sometimes, by measuring the two different components of total bilirubin, namely the unconjugated and conjugated bilirubins, we can try to determine the location of the problem.
The other liver enzymes previously mentioned will help give us further ‘clues’ in the interpretation of bilirubin changes. For example, if the ALT enzyme is markedly increased, it suggests that there is some damage within the liver that 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).
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.
Since the kidneys constantly filter the blood to remove toxins and other wastes, we may occasionally see changes in the urinalysis before significant changes are noted with 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 the 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 to us for the diagnosis of liver disease?
Serum bile acid concentrations 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 will be re-absorbed into the blood from the intestines and are subsequently removed from the circulation by the liver (this is termed enterohepatic circulation of bile acids).
This normal enterohepatic 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 2 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.
If liver function is reduced for any reason, the concentration of bile acids in circulation may be increased in either the post-feeding sample or possibly in both samples. Occasionally the difference between the fasting and post-feeding samples may suggest specific disease processes. Usually increases, especially in the post-feeding samples, only provide further evidence of liver disease.
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 through clinical history and physical examination will suggest a likely cause or pathogenesis to the liver disease.
However, in most instances further testing is required. This testing may include ultrasound examination of the liver and gall bladder. An analysis of any free fluid in the abdominal cavity that might be detected on ultrasound evaluation, and biopsy or fine needle aspiration of the liver and/or gall bladder may be performed at this time if abnormalities are observed. Since the liver produces the proteins involved in blood clotting, we will need to determine the amount and functional activity of these proteins before proceeding with any biopsies.
A sample of 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 titre(See handout on antibody titres for further information).
If these other tests are needed to determine what kind of liver disease is present, why bother with the screening tests at all?
Firstly, these screening tests allow us to document whether or not your pet has liver disease. These screening tests are minimally invasive and of much lower cost than the biopsies and ultrasound evaluations discussed above.
Secondly, these tests will allow us to monitor how your pet is responding any treatments that we undertake. Normalization of liver enzymes is a good indicator of response to treatment.