The liver is a vital organ that serves numerous essential functions, including filtering out toxins in the bloodstream, manufacturing proteins and fats, storing glycogen (an important source of energy), and producing bile, a fluid that aids digestion. Dogs and cats suffering from liver disease may exhibit clinical signs, but a liver test is mandatory for determining the nature of the disease. Pet owners do not always know what these tests imply or what the results mean. Knowing how to interpret the results of your pet’s bloodwork testing will educate you to knowing what is wrong and how to work with your veterinarian to fix it.
What factors affect liver function?
Since the liver is the primary detoxifying organ in the body, there are many things that can affect liver function. They include:
- Environmental toxins: Chemical cleaners, pesticides, herbicides, and flame retardants (on furniture and carpets), whether inhaled or absorbed through the skin.
- Vaccines: additives, adjuvants, and preservatives.
- Drugs: Any medication given to your pet will be filtered by the liver. This includes chemical parasite preventatives (flea & tick), heartworm preventative and any other over-the-counter or prescribed drug.
- Processed foods: filled with chemical additives like food dyes and euthanasia solution.
- Viruses: Viral infections can affect the liver.
- Autoimmune disease: Autoimmune liver disease develops when your pet’s immune system mistakes normal, healthy liver tissue for a foreign body.
- Cancer: Liver cancer or liver damage caused by chemicals (such as those used in cancer medicines) is called hepatotoxicity.
- Mycotoxins: Dry kibble is often filled with mold toxins (from peas, corn, or other grains).
- Shunts: Because a liver shunt bypasses the liver partially or completely, the liver is not filtering at its capacity, resulting in complications.
- Pollution: Water and air pollution can lead to a cascade of problems within the liver, including inflammation, stress responses, cell damage, and disrupted energy metabolism.
- Stress, frustration, boredom, or overwork.
What are the most common tests used to screen for liver problems?
The most common liver values are reported on routine screening tests that pets should receive on at least an annual basis. These tests support the presence of liver disease or a reduction in the functional capacity of the liver. They are not necessarily specific for the type of liver disease. These tests use blood to evaluate liver enzymes, albumin, cholesterol, and serum bile acid concentrations. The most common tests are:
- A Complete Blood Count analyzes the cellular components of the blood.
- A serum biochemical profile (or “chem panel”) is a chemical analysis of the liquid component of blood.
- A Urinalysis tests the chemical properties of the urine.
If a pet does not show symptoms of liver problems, these tests are often the first indication of a problem. Common symptoms of liver disease include:
- Poor appetite and/or weight loss
- Excessive drooling
- Increased thirst – often one of the first symptoms
- Nausea, vomiting or diarrhea
- Jaundice
- Changes in behavior because of ammonia build up in the blood stream.
- Seizures
- Inflammation of the eyes
- Dry, flakey skin
- Tendon and ligament damage
What are the most common liver values used to diagnose liver problems?
Important liver values to monitor include four hepatic enzymes (ALT, AST, ALKP, and GGT), as well as cholesterol, albumin, bilirubin, and bile acids. An increased value in any of these tests can signal a liver problem. Changes in the biochemistry profile do not tell us what type of liver disease is present but may suggest a certain category of liver disease. How they relate to each other as well as values from other tests can be complicated.
ALT – Alanine Aminotransferase
ALT is a liver enzyme found within individual liver cells. When liver cells are damaged, ALT will be released into the bloodstream. Drugs, especially steroid-containing drugs and non-steroidal anti-inflammatory drugs (NSAIDs) may increase ALT values. Anti-seizure drugs such as phenobarbital can also raise ALT. The amount of increase in the ALT value can determine whether your pet should take this type of medication. Certain diseases can increase ALT including endocrine disorders such as diabetes and Cushing’s disease, inflammatory bowel disease (IBD), pancreatitis, cholangiohepatitis (inflammation of the liver, gallbladder, and bile ducts), cancer, hyperthyroidism, hepatic lipidosis (fatty liver disease), and severe muscle necrosis (dying cells have died off due to trauma).
AST – Aspartate Transaminase
Like ALT, AST is a liver enzyme found in liver cells. AST is released when the liver is damaged. AST is also found in muscle cells, and therefore, increases in this enzyme are not liver specific. This elevation is often found in animals who have blood drawn soon after a great deal of physical activity or trauma. Steroids do not result in significant increases in this enzyme. Diseases that cause elevation of AST include muscle necrosis, heart failure, strenuous exercise, and primary liver disease.
ALKP – Alkaline Phosphatase
ALKP is a group of enzymes found on the outer lining of liver and bile duct cells. ALKP can be elevated for several reasons. Liver conditions that result in obstruction of the bile flow (cholestasis) may result in increased serum ALKP concentrations. While the reason in elevation can be traced to the liver, ALKP can also go up with a female suffering from pyometra (uterine infection). Pets with endocrine diseases such as diabetes and Cushing’s disease may have higher ALKP levels. Other diseases that may cause ALKP elevation include Osteomyelitis (bone infection), Osteosarcoma (bone cancer), secondary renal hyperparathyroidism (increased levels of parathyroid hormone because of long-term kidney disease), and liver cancer. Swelling in the bile ducts or liver will also result in elevated levels of ALKP. Other causes include certain drug therapies, especially steroids and anticonvulsants, uterine infections, swelling in the bile ducts or liver, and dogs with rapidly growing bones. Additional testing may be required to determine why ALKP is elevated.
GGT – Gamma-Glutamyl Transferase
GGT is found in the cell membranes of the bile ducts, kidneys, and pancreas. An increase of GGT results from liver conditions that result in cholestasis (impaired bile flow), cholecystitis (inflammation of the gallbladder), primary liver cancer, aflatoxicosis or other mycotoxins (poisoning from mold toxins found in food), and steroid-containing and anticonvulsant drugs. The degree of GGT elevation will determine whether an animal can continue using these drugs. If elevated GGT activity is noted along with an increase in serum ALKP, a diagnosis of liver disease increases to 94%. If ALKP is elevated but GGT is not, then skeletal disease is more likely the problem. Some blood panels do not include GGT, so be sure to ask your veterinarian to run the panel that includes it.
Tbil – Total Bilirubin
In healthy animals, red blood cells are normally removed from circulation when they become aged (about 2 weeks). Once removed, they break down into several components, one of which is bilirubin. Bilirubin is processed in the liver and passed to the intestines as a component of bile. Too much bilirubin in the blood can result from an increased destruction of red blood cells or a 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. This value must be considered along with other values in the blood work. For example, if Tbil is high but red blood cell count is normal, further testing would focus on liver problems. If other liver enzymes are normal but the animal is anemic, it could signal an immune mediated disease such as acute hemolytic anemia. If ALT is markedly increased alongside higher Tbil, it suggests that damage within the liver is contributing to the problem of bilirubin processing. If GGT is elevated alongside higher Tbil, there could be a problem with the excretion of bilirubin from the liver into the bile ducts and gallbladder (cholestasis).
Chol – Cholesterol
Cholesterol is made by the liver. We commonly do not worry about high levels of cholesterol in animals as we do with humans. However, high levels can signal hypothyroidism. High cholesterol is also common in overweight dogs. If cholesterol is high, thyroid levels should also be checked. High cholesterol is often a by-product of another endocrine disease (diabetes, hyperadrenocorticism, and nephrotic syndrome). Low levels of cholesterol can indicate decreased liver function, cirrhosis of the liver, end stage liver disease, hypoadrenocorticism, and malnutrition/starvation. Cirrhosis of the liver can be caused by long term use of anti-convulsant drugs such as phenobarbital.
Alb – Albumin
Total protein is made up of albumin and globulin. Globulins are made by plasma cells and are part of the animal’s immune system. Albumin is made by the liver and makes up more than half of the total protein present in blood serum. It serves in the transport of bilirubin, hormones, metals, vitamins and drugs. Albumin has an important role in fat metabolism by binding fatty acids and keeping them in a dissolvable form in the bloodstream. Albumin binds to calcium. Calcium will be low if albumin is low. Causes of low albumin are decreased production due to severe liver disease, maldigestion (EPI) Exocrine pancreatic insufficiency (liver cannot absorb nutrients from food), portosystemic shunting, and inflammation. Diseases associated with an increased loss of albumin also include protein-losing nephropathy (PLN -losing albumin through the urine), protein-losing enteropathy (PLE- losing protein through the digestive tract), and Addison’s disease. Symptoms of low albumin include swelling of extremities, such as legs and paws, vomiting, weakness, diarrhea, distended abdomen, and breathing difficulty.
SBA - Serum Bile Acid
A serum bile acids test is a special test used to evaluate the functional capacity of an animal’s liver. This test is not performed as part of a routine blood chemistry panel and requires a 12-hour fast before blood is drawn. Bile acids are a major component of bile, which is stored in the gallbladder and released into the intestines to aid in digestion. After assisting in digestion, bile acids are re-absorbed into the blood from the intestines and are subsequently removed from circulation by the liver. It is this circulation that forms the basis for the serum bile acid test. Increased SBA can help diagnose portosystemic shunting, hepatic parenchymal (liver tissue) disease, and bile duct obstruction.
The results of the tests listed above can prompt additional diagnostic testing such as ultrasound and biopsy; individual liver function values should always be analyzed in combination with each other. Your veterinarian can help you fit the pieces of the puzzle together to determine if there are any problems. The good news is that a dog’s liver can function even when 80% is consumed by disease. The liver is an organ that can regenerate. However, it is best to catch problems early. The results of these tests can signal liver problems even before symptoms begin. Liver problems are very common in dogs and cats – by staying up to date with regular blood testing, potential issues can be dealt with before they turn into major problems.
For more information, check out the following!
https://www.youtube.com/watch?v=RZoygZTZ0WI
