Ammonia
Waste product of protein metabolism that the liver converts into urea
Also known as
Definition
Ammonia (NH3) is a metabolic waste product; among other sources, gut bacteria make it when they break down protein from food. The liver converts ammonia into urea in the urea cycle, and the kidneys excrete urea in urine. If this fails, ammonia builds up in the blood. Because it is toxic to the brain, the level is measured mainly in liver disease, unexplained confusion and suspected inherited metabolic disorders.
| Parameter | Value |
|---|---|
| Unit | µmol/l (in the US µg/dl) |
| Reference Range | Typically about 11–32 µmol/l (15–45 µg/dl) in adults. The range depends on age, method and sample type. Labs differ; the range printed on your report is what counts, and your doctor interprets the result. |
Agree on your personal target range with your doctor.
Conversion: µg/dl × 0.714 = µmol/l. MedlinePlus gives 15–45 µg/dl or 10.7–32 µmol/l; the factor 0.714 corresponds to reporting as ammonia nitrogen.
↓ What a low value means
Low ammonia levels are generally considered unremarkable. The body normally keeps the concentration in the blood low because the liver continuously converts ammonia into urea. MedlinePlus does not describe a specific disease meaning for low levels.
↑ What a high value means
A raised level shows that the body is not clearing ammonia adequately or is producing more of it. The most common cause is liver disease such as cirrhosis or hepatitis. Raised levels can also be linked to kidney disease, inherited urea cycle disorders, bleeding in the upper digestive tract, the rare Reye syndrome, certain medicines, heavy muscle exertion or a very high-protein diet. Confusion, unusual sleepiness or vomiting together with a raised level should be assessed by a doctor right away. Your doctor will investigate the cause with further tests.
When to test
When symptoms could point to too much ammonia (confusion, sleepiness, vomiting, loss of coordination), to monitor known conditions, and when urea cycle disorders run in the family. Do not smoke before the blood draw; your practice will tell you whether to fast and avoid exercise or alcohol. Never stop medicines on your own.
Frequently asked questions
Is ammonia part of a routine check-up? +
Usually not. Ammonia is measured specifically when symptoms or existing conditions such as liver disease suggest a raised level. For a general overview of the liver there are separate liver values.
How is ammonia related to urea? +
The liver converts ammonia into urea in the urea cycle. Urea is far less toxic and is excreted by the kidneys in urine. Both values therefore describe different stages of the same clearance pathway.
Can exercise or diet affect the level? +
Yes. Heavy muscle exertion and a very high-protein diet can raise ammonia levels. Smoking, alcohol and some medicines can also affect the result. That is why your practice gives you preparation instructions before the test.
Why is blood sometimes taken from an artery? +
Blood is usually drawn from a vein in the arm. Some doctors consider arterial blood more accurate and then take the sample from an artery, for example at the wrist. This is somewhat more uncomfortable than a normal blood draw.
Ammonia
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.