Bilirubin is a yellow-orange pigment produced during the normal breakdown of old red blood cells. In a healthy cat, the liver processes this bilirubin and excretes it into the bile, which eventually leaves the body through the feces. Finding bilirubin in a cat's urine, known as bilirubinuria, is always considered an abnormal finding that requires immediate investigation.
The "Feline Rule" of Bilirubin
It is important to note a major difference between cats and dogs regarding this marker:
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In Dogs: A small amount of bilirubin in concentrated urine can be normal.
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In Cats: The feline kidney has a very high "renal threshold" for bilirubin. This means it is very difficult for bilirubin to leak into the urine. If it appears on a dipstick, it means the levels in the blood are significantly elevated. Any amount of bilirubin in feline urine is considered clinically significant.
Why Bilirubin Appears in CKD Cats
While bilirubin is primarily a "liver" marker, it often appears in cats with Chronic Kidney Disease due to related complications:
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Hepatic Lipidosis (Fatty Liver): If a CKD cat stops eating due to uremic nausea, their body begins moving fat stores to the liver for energy. Cats are very poor at processing this fat, leading to a "clogged" liver that cannot process bilirubin, which then spills into the blood and urine.
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Red Blood Cell Breakdown (Hemolysis): If red blood cells are being destroyed faster than normal (due to toxins, certain medications, or infections), the liver becomes overwhelmed with the sudden "dump" of bilirubin.
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Biliary Obstruction: Inflammation or "sludge" in the gallbladder (common in older cats with "triaditis") can prevent bile from flowing, forcing bilirubin back into the bloodstream.
The Impact on the CKD Cat
Bilirubin itself is not usually toxic to the kidneys in the same way that protein or glucose can be, but its presence signals a "systemic crisis" that the kidneys must deal with:
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Indicator of Anemia: Since bilirubin comes from red blood cells, its presence can sometimes explain why a CKD cat's anemia is worsening—it points toward the destruction of cells rather than just a lack of production.
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Dehydration and Nausea: Liver issues cause profound nausea, which makes the CKD cat stop drinking and eating, leading to a rapid "renal crash" from dehydration.
Interpreting the Dipstick
On a urine dipstick, the "Bilirubin" pad should stay a light cream or beige color.
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Any Color Change (1+ to 3+): This is a red flag. Even if the cat’s gums don't look yellow yet (jaundice), the urine will show bilirubin first.
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False Positives: Certain medications or very dark, concentrated urine can occasionally cause a slight color shift, but in a cat, vets will almost always assume it is a true positive until proven otherwise.
The Diagnostic Path
If bilirubin is found in the urine of a CKD cat, the veterinarian will typically:
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Check the Bloodwork: Look at the ALT, GGT, and Total Bilirubin on the chemistry panel to evaluate liver health.
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Check the PCV/HCT: Ensure the cat isn't suffering from hemolytic anemia.
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Abdominal Ultrasound: This is the best way to look at the physical structure of the liver and gallbladder to check for inflammation or "sludge."
Management
Management focuses on treating the underlying cause, whether that is starting an appetite stimulant to prevent fatty liver, providing liver-supportive antioxidants (like SAMe or Silybin), or addressing an infection.
Disclaimer: This content is for educational purposes only and does not constitute veterinary medical advice. Always consult with your primary care veterinarian or a veterinary internal medicine specialist regarding your cat’s specific condition.