Ammonia in cats and dogs

What it shows

Ammonia comes from amino acids in food and from protein breakdown in the body, especially in muscle; bacteria in the large intestine add more. It is absorbed into the portal vein and carried to the liver: of all the ammonia produced, 80–90% enters the urea cycle there. Ammonia is toxic to the brain and is one of the factors behind hepatic encephalopathy — brain dysfunction caused by liver disease — though not its only cause.

The test mainly shows whether blood from the gut actually reaches working liver cells. That is why it reacts to shunts and to severe loss of liver function, but is not influenced by blocked bile flow or by liver diseases that do not disturb blood flow from the portal vein to the liver.

Higher than reference — what it may be a sign of

• Sample handling — the most common cause of a high result: late delivery, late separation of plasma, hemolysis. If whole blood is not kept cold, ammonia leaks from red blood cells within 30 minutes and the value comes out falsely high. Contamination from sweat or cigarette smoke can also raise it.
• Blood bypassing the liver — a congenital or acquired portosystemic shunt.
• Liver failure — ammonia can rise once more than 60–70% of liver function is lost.
• Causes outside the liver: a high-protein meal or strenuous exercise shortly before the test; a bacterial urinary tract infection, the most common non-liver cause; cobalamin (B12) deficiency; rare inherited defects of the urea cycle.

Lower than reference

A low ammonia is not used to diagnose anything. A falsely low result happens when the tube was not completely filled or not stoppered, so ammonia escaped as gas.

More important is the opposite trap: a normal ammonia does not rule out a liver problem. Animals with obvious hepatic encephalopathy may have normal blood ammonia, and a single result inside the range cannot exclude it in an animal with suspected chronic liver disease.

Look at together with

• Bile acids — the main companion test. In dogs with congenital shunts and signs, fasting ammonia was less sensitive than fasting bile acids, but a high ammonia together with high fasting bile acids was specific for a shunt.
• Urea — the substance the liver makes from ammonia.
• Cobalamin (B12) — its deficiency is one of the non-liver causes of high ammonia in dogs and cats.
• Urine sediment — the appearance or disappearance of ammonium biurate crystals is one of the few lab signs that follows changes in hepatic encephalopathy.

Units and conversion

Ammonia is reported in µmol/L or in conventional units, µg/dL. To convert, multiply µg/dL by 0.587: 100 µg/dL is about 59 µmol/L. In the app each row keeps the unit printed on the form; the chart converts to one unit so forms from different labs line up.

Reference range

The reference range is on your lab’s form, and for ammonia it is meaningful only if the sample was handled correctly. Blood is collected into EDTA (preferred) or heparin, kept on ice and processed fast: eClinPath advises separating plasma within 15 minutes, and Merck advises taking the pre-cooled sample on melting ice to the analyser within 20 minutes. Because ammonia is volatile, the sample cannot simply be mailed; when a delay is unavoidable, plasma is frozen and shipped on dry ice so it does not thaw. Because of this instability, repeated measurements are not a reliable way to follow hepatic encephalopathy. There is also a provocation test, the ammonia tolerance test; it is seldom used.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine) — Ammonia
2. Hepatic Function Tests in Small Animals. Merck Veterinary Manual
3. Hepatic Encephalopathy in Small Animals. Merck Veterinary Manual