Ammonia

Ammonia is a toxic byproduct of protein metabolism, produced primarily by bacteria in the intestines during the breakdown of proteins. In a healthy cat, the liver quickly converts ammonia into urea (BUN), which is then excreted by the kidneys. In a cat with Chronic Kidney Disease (CKD), ammonia levels are monitored to assess the "cross-talk" between the liver, the gut, and the brain.

The Conversion Cycle

• Production: As your cat digests protein, ammonia is created in the gut.

• The Liver’s Job: The liver "grabs" this ammonia from the blood and turns it into the less toxic substance, urea.

• The Kidney’s Job: The kidneys filter that urea out of the body.

• The Failure: If the liver is overwhelmed (as seen in Hepatic Lipidosis or Cardiorenal Syndrome) or if the blood bypasses the liver, ammonia levels rise.

Why Ammonia is Relevant for CKD Cats

While ammonia is often thought of as a "liver value," it has a profound impact on the symptoms of a CKD patient:

• Uremic Encephalopathy: High levels of uremic toxins from failing kidneys can impair the liver's efficiency. If ammonia builds up alongside these toxins, it can cross the "blood-brain barrier," causing neurological symptoms like pressing the head against walls, staggering, or seizures.

• The Protein Paradox: CKD cats need protein restriction to keep their BUN low. However, if they have concurrent liver issues, they need protein restriction to keep Ammonia low as well. Finding a diet that satisfies both needs without causing muscle wasting is a major challenge.

• Bacterial Overgrowth: Many CKD cats have altered gut motility. This can lead to an overgrowth of ammonia-producing bacteria in the intestines, which adds to the toxic load the liver and kidneys must process.

High Ammonia (Hyperammonemia)

In a renal cat, elevated ammonia is a medical emergency. It is often caused by:

• Hepatic Lipidosis (Fatty Liver): If a CKD cat stops eating, the liver becomes too fatty to process ammonia.

• Severe Constipation: If stool sits in the colon for too long (common in dehydrated CKD cats), bacteria have more time to produce ammonia, which is then reabsorbed into the blood.

• Portosystemic Shunts: Rare in older cats, but a shunt allows blood to skip the liver entirely, leaving ammonia to circulate freely to the brain.

The Difficulty of the Test

The ammonia test is notoriously difficult to perform accurately:

• The "Cold" Requirement: Ammonia is very unstable. The blood sample must be placed on ice immediately and run within 15 to 30 minutes.

• The "False Positive": If the sample sits at room temperature or the cat struggled during the blood draw (muscle exertion), the ammonia level will appear falsely high.

Interpreting the Results

Ammonia is typically measured in µmol/L.

• Normal: Generally less than 60 µmol/L.

• Abnormal: Values above 100 µmol/L are concerning.

• Critical: Above 200 µmol/L, neurological signs (Hepatic/Uremic Encephalopathy) are almost always present.

Management

• Lactulose: This is the primary treatment. It's a syrupy laxative that changes the pH of the gut, "trapping" ammonia so it is pooped out rather than absorbed.

• Antibiotics (Metronidazole): These can be used to kill off the specific bacteria in the gut that produce the most ammonia.

• Hydration: IV fluids help dilute toxins and ensure the liver has the blood flow it needs to function.

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.