Diseases of the liver occur commonly in cats. Hepatic lipidosis (“fatty liver disease”), hepatitis, feline infectious peritonitis, and liver cancer are some of the more common diseases that can affect the feline liver. Trying to make a proper diagnosis can be challenging for many veterinarians. Routine blood tests often yield information that the liver is affected, but rarely gives enough information to diagnose the exact nature of the liver problem. Other diagnostic tests (x-rays, ultrasound, etc.) are necessary to further characterize the liver condition. Ultimately, a biopsy of the liver is often the only way to make a definitive diagnosis.

Deciding if and when to perform a liver biopsy has been a concern for veterinarians. Although liver specimens may be obtained through an ultrasound-guided biopsy or via laparoscopy, these methods do not necessarily guarantee that the specimen obtained is of adequate size or is a truly representative sample of the compromised liver. These techniques also may not be available to every veterinarian. For many vets, the only way to obtain an adequate biopsy specimen is via exploratory surgery, an invasive and potentially expensive procedure. Exploratory surgery requires general anesthesia. Some of the anesthetics commonly used in veterinary medicine require normal liver function for metabolism, making anesthesia challenging and potentially risky.

Detection of abnormal liver enzyme levels alone may or may not justify a liver biopsy. The commonly measured liver enzymes alert veterinarians to the possibility of liver disease, but often do not allow us to make judgments about liver function, per se. To justify a biopsy, we need evidence that liver function is impaired. In the past, liver function tests were cumbersome, often requiring multiple blood sampling throughout the day, or requiring special handling of samples. About 15 years ago, a new test was introduced whereby liver function could be assessed by measuring serum bile acid levels.
The liver makes bile acids. These bile acids are stored in the gall bladder. When cats eat a meal, the gall bladder contracts, releasing these bile acids into the intestinal tract, where they help in the digestion of food. Bile acids that are released into the intestinal tract are absorbed into the blood stream, and as they pass through the liver, the liver grabs them from the blood stream and puts them back into the gall bladder, to be recycled. The normal liver does this rapidly the first time the bile acids pass through.

To perform a serum bile acid test, the cat is fasted overnight. The next day, a blood sample is taken. The cat is then given a meal, so as to cause the gall bladder to contract. Two hours later, a second blood sample (the “post-prandial” sample; post prandial means “after eating”) is obtained. Elevations in the post-prandial level of bile acids indicate that the liver is not able to clear the bile acids from the bloodstream and is therefore not functioning properly. Abnormal liver function is justification for a biopsy.

Sometimes, interpretation of the bile acid levels can be perplexing. Sometimes, the fasting bile acid levels are higher than the post-prandial levels, and this has always been confusing to interpret. Also, the fact that some cats do not tolerate repeated attempts at having blood taken, has led some practitioners to adopt the practice of using fasting bile acids alone as a screening test, to be interpreted in conjunction with routine blood chemistry analysis. Unfortunately, fasting bile acids alone are not very sensitive in detecting liver disease.

It has been suggested that determination of bile acids accumulating in urine over time would be a good alternative approach. Studies of urine bile acids in humans have led to the suggestion that these bile acid levels reflect the “average” amount of bile acids in the circulation during the interval of time that the urine was formed.

Researchers at Cornell University, Ithaca NY, Oradell Animal Hospital, Oradell NJ, and Kyoto Research Laboratories, Kyoto Japan collaborated to study whether measuring bile acids in cat urine was as useful, clinically, as measuring serum bile acids to detect liver disease. The study, published in the March/April 2003 issue of the Journal of Veterinary Internal Medicine, indeed has shown the measuring of urine bile acids to be a convenient diagnostic test for identifying cats with clinically significant liver disease. The test may also have use as a screening test for identifying cats with subclinical liver disease. As further research in this area continues, practitioners may soon have a simple urine test at their disposal, allowing us to detect liver dysfunction at earlier stages than ever before, without having to fast the cat overnight or draw repeated blood samples. The earlier we can detect a problem, the greater the chance of curing, controlling, or slowing its progression. Its tests like these that are responsible for cats living longer, healthier lives, and we tip our hats to the veterinarians and scientists who devote themselves to this kind of research.

Updated 2/9/06