Blood ammonia: what it is, what it tells you and when it’s tested

Blood ammonia is the amount of ammonia in your blood. Ammonia forms every day, mostly in the gut, as you digest protein. The liver turns it into urea, which leaves in your urine. If the liver cannot keep up, ammonia builds up and can reach the brain. It is a test almost always done in hospital: if someone is confused or very drowsy, call 112 or go to the emergency department.

When it’s tested

Doctors mostly order it when someone is confused, disoriented or very drowsy with no clear reason. In people with cirrhosis, it helps work out whether the liver is the cause.

It is also measured in people taking some medicines, like valproate, who become confused, and when a rare inherited metabolic condition is suspected. These conditions usually show in children, but sometimes appear in adults.

It is not a test for people who feel well. The sample is delicate: the tube must go on ice and be processed straight away, or the value rises on its own. That is why Rosa does not test it. Instead it measures ALT and AST, gamma GT, bilirubin, albumin and platelets, which show how your liver is doing.

What it tells you, and what it does not

In a confused person with liver disease, a normal ammonia makes it unlikely that the confusion comes from the liver. The doctor then looks for another cause.

A high value, on the other hand, is not enough for a diagnosis on its own and does not say how serious things are. Doctors look mainly at the symptoms and the examination.

That is why, in people with cirrhosis, it is usually not repeated to follow treatment. How the person is doing is what counts.

That is why it is read together with other tests:

• ALT and AST: They show whether the liver is inflamed
• Albumin: It falls when the liver makes less
• Bilirubin: It rises when the liver clears less
• Platelets: In cirrhosis they are often low
• Urea: Urea is what the liver turns ammonia into

Normal blood ammonia levels

The lab decides what is normal, and it varies a lot: it depends on the method, differs between men and women, and is higher in newborns. Some reports use µmol/L, others µg/dL, with very different numbers. So there is no single number for everyone: the range on your report is what counts.

How the sample was taken matters more than the number. If the tube did not reach the lab straight away on ice, the value can be high even though the ammonia in your blood was normal.

If your blood ammonia is high or low

• High blood ammonia: It usually comes from the liver. Sometimes it is just a badly handled sample. Your doctor reads it together with your symptoms.
• Low blood ammonia: It has no clear meaning. Usually nothing needs doing.

What raises it

The most common causes come first.

• Cirrhosis and other liver conditions: The liver turns less ammonia into urea, and some blood bypasses it.
• A trigger, in cirrhosis: An infection, bleeding in the stomach or gut, constipation, dehydration or some sedatives can make it rise suddenly.
• Acute liver failure: A liver that stops working within days, for example because of a medicine or hepatitis. It is an emergency.
• Some medicines: Valproate, a medicine for epilepsy and mood, is the best known. Do not stop it on your own: talk to your doctor.
• Inherited metabolic conditions: Rare. Sometimes they are found in adulthood, after an illness, an operation or a very protein-rich meal.
• A badly handled sample: A tourniquet left on too long, clenching and opening the fist, a tube not put on ice or processed late: all of these raise the value.

If you have cirrhosis

A few rules help keep ammonia under control:

• Take the medicines you have been prescribed, such as lactulose, every day. Do not stop them on your own.
• Try to open your bowels regularly: constipation raises ammonia. Your doctor will tell you how.
• Do not cut down on protein on your own. In cirrhosis you need it, so you do not lose muscle. Ask your doctor or dietitian how much.
• Avoid alcohol, and do not take sleeping pills or sedatives without asking your doctor.

The people you live with should know the first signs of confusion. If you are confused or very drowsy, they should call 112.

What can skew the result

The value rises easily for reasons that have nothing to do with the liver.

• A tube not on ice: Blood outside the body keeps making ammonia. The tube has to be cooled and processed straight away.
• Tourniquet and fist: A tourniquet left on too long, or opening and closing your fist, can raise it.
• Hard exercise: A hard workout shortly before raises it, because muscles make ammonia too.

When to worry

Watch for these cases:

• Someone, especially with liver disease, is confused, disoriented, very drowsy or behaving strangely: call 112 or go to the emergency department.
• Someone with cirrhosis vomits blood or has black, tar-like stools: call 112 or go to the emergency department.
• You take valproate and become confused, very drowsy or start vomiting: call 112 or go to the emergency department.
• You have a report with high blood ammonia and you feel well: talk to your doctor. It is often repeated with a properly handled sample.

Frequently asked questions

What are normal blood ammonia levels? They vary a lot between labs, between men and women and with the unit used. The range on your own report is what counts.

Do I need to fast for a blood ammonia test? You are often asked not to eat, smoke or exercise beforehand. Follow your lab’s instructions.

High blood ammonia: what does it mean? Usually that the liver is not clearing ammonia well, mostly in people with cirrhosis. Sometimes it is just a badly handled sample. If there is confusion, call 112.

How is blood ammonia linked to hepatic encephalopathy? Hepatic encephalopathy is the confusion that comes from a diseased liver. Ammonia plays a part, but the blood value alone does not make the diagnosis. If it is normal, the doctor looks for another cause.

Can any lab do a blood ammonia test? Not always. The sample must go on ice and be processed straight away, so many labs only do it under strict rules. It is usually done in hospital.

Does Rosa test blood ammonia? No. It is a hospital test. Rosa measures ALT and AST, gamma GT, bilirubin, albumin and platelets, which show how your liver is doing. If your doctor wants blood ammonia, they order it separately.

Sources

• Vilstrup and colleagues, hepatic encephalopathy in chronic liver disease: 2014 practice guideline by AASLD and EASL. Hepatology, 2014. PMID 25042402
• European Association for the Study of the Liver, clinical practice guidelines on the management of hepatic encephalopathy. Journal of Hepatology, 2022. PMID 35724930
• Häberle and colleagues, suggested guidelines for the diagnosis and management of urea cycle disorders: first revision. Journal of Inherited Metabolic Disease, 2019. PMID 30982989