Ammonia Blood Test

What Does This Test Measure?

Ammonia is a neurotoxic waste product of protein metabolism. The liver converts ammonia to urea (via the urea cycle) for excretion by the kidneys. Elevated ammonia primarily indicates liver dysfunction and is responsible for hepatic encephalopathy, the confusion and altered mental status seen in severe liver disease.

Why Your Doctor Ordered This Test

Ammonia testing is ordered when a patient presents with unexplained confusion, lethargy, or altered consciousness, especially in the context of known or suspected liver disease. The cardinal clinical scenario is hepatic encephalopathy, the neuropsychiatric syndrome that ranges from subtle personality changes to deep coma, triggered by ammonia accumulation when the liver cannot detoxify it. Emergency departments measure ammonia when cirrhosis patients arrive with acute mental status changes, as levels above 100–150 µg/dL correlate with encephalopathy and guide therapy. In the neonatal nursery, ammonia is measured urgently in any infant with poor feeding, vomiting, seizures, or lethargy to rule out inborn errors of the urea cycle. These genetic defects, like ornithine transcarbamylase deficiency, can cause catastrophic hyperammonemia within days of birth. Doctors also order serial ammonia measurements when monitoring patients on valproic acid (a medication for seizures or bipolar disorder) because valproate can raise ammonia even when standard liver enzymes remain normal. In oncology, high-dose chemotherapy, particularly for leukemia, can provoke hyperammonemia from tumor lysis or drug toxicity, prompting ammonia surveillance to prevent neurological injury.

Beyond acute mental status shifts, the test is part of the workup for suspected Reye syndrome, a rare but severe illness linking aspirin use during a viral febrile illness in children to vomiting, liver failure, and elevated ammonia. It is also ordered when a patient with portosystemic shunting (for example, after a transjugular intrahepatic portosystemic shunt, TIPS, procedure for portal hypertension) develops subtle cognitive complaints, as shunts allow ammonia-rich blood to bypass the liver. In the intensive care unit, unexplained hyperammonemia can be a clue to a missed liver shunt, an occult urea cycle disorder presenting in adulthood, or a side effect of multiple medications. Ultimately, the test helps doctors differentiate between metabolic, toxic, and structural liver causes of encephalopathy and to gauge the severity and guide lactulose or other ammonia-lowering therapies.

What High Ammonia Means

High ammonia (hyperammonemia): severe liver disease (cirrhosis with portosystemic shunting, blood bypasses the liver, and ammonia reaches the brain), acute liver failure, Reye syndrome (aspirin in children with viral illness), urea cycle disorders (rare genetic, often present in infancy with severe hyperammonemia), and certain medications (valproic acid; can cause hyperammonemia even with normal liver function; carbamazepine, high-dose chemotherapy). Ammonia >100–150 µg/dL is associated with hepatic encephalopathy.

What Low Ammonia Means

Low ammonia: not clinically significant. No recognized disease state is associated with low ammonia. Levels below detection are normal.

How to Prepare and What Affects the Result

Proper collection and handling are critical because ammonia concentrations rise rapidly in blood after drawing. The sample must be drawn into a chilled tube (usually a heparinized tube placed on ice) and transported to the laboratory immediately for processing within 15–20 minutes. A tourniquet applied for more than one minute can artificially elevate ammonia due to local metabolic activity, so the phlebotomist should release the tourniquet before drawing blood. The patient should be advised to avoid smoking for at least 8 hours before the draw because cigarette smoke delivers ammonia directly into the bloodstream. Recent high-protein meals, excessive exercise, or physical stress can transiently raise ammonia, so fasting for 8–12 hours is recommended, though emergency testing often overrides this requirement. Certain medications markedly distort ammonia results. Valproic acid (valproate) is the most notorious; even at therapeutic doses, it can cause hyperammonemia independent of liver function, and levels should be checked if the patient is on this drug. Carbamazepine, high-dose corticosteroids, and some chemotherapy agents also raise ammonia. Conversely, lactulose, rifaximin, or other ammonia-lowering therapies will lower the value. Providers should note any recent contrast dye exposure (which can interfere with colorimetric assays) and inform the lab of all medications.

For neonates, specimen requirements are just as stringent. A heel-stick sample must be collected without squeezing, as muscle breakdown releases ammonia. The sample is immediately iced and sent to the lab, with a stat turnaround time. In any age group, avoid hemolysis, which releases intracellular ammonia. If the patient arrives in a state of acute encephalopathy, clinicians may draw before initiating therapy but must still adhere to cold processing to avoid a falsely normal or falsely high result that could misdirect care.

Common Misinterpretations

The most frequent misinterpretation of an elevated ammonia level is attributing it to liver failure when the cause is a preanalytical error. Ammonia is notoriously unstable ex vivo; a sample left at room temperature for more than 20 minutes will show spuriously high results because red blood cells and plasma enzymes generate ammonia. Clinicians unfamiliar with phlebotomy protocol may see a value of 80 µg/dL and diagnose early hepatic encephalopathy when, in fact, the specimen was mishandled. Likewise, a tourniquet kept on for two minutes or more raises ammonia by local anaerobic metabolism, producing a false-positive hyperammonemia. In the emergency setting, a patient who recently smoked a cigarette or ate a steak can present with an ammonia level of 60–70 µg/dL that falls within normal limits by the next morning’s fasting draw, yet gets mislabeled as a liver problem. Recognizing these artifacts requires correlating the ammonia level with the clinical picture and, if doubt exists, repeating the test with strict cold-chain handling.

Another common misinterpretation involves valproic acid therapy. Many physicians assume that a normal ALT and AST exclude drug-induced hyperammonemia, but valproate raises ammonia through inhibition of the urea cycle and increased renal production, pathways independent of hepatocellular injury. Thus, a patient on valproate with an ammonia of 120 µg/dL may have no liver enzyme elevations and can still develop clinically significant encephalopathy. Conversely, lowering plasma ammonia with lactulose in such cases without adjusting valproate dose is a mistake. Additionally, using a single absolute threshold to diagnose encephalopathy is problematic: chronically cirrhotic patients may tolerate ammonia levels of 150 µg/dL without symptoms, while acute liver failure patients might develop brain edema at lower levels. Ammonia values must be interpreted alongside mental status, liver function tests, and imaging. Falsely low ammonia can also occur if the sample was processed too quickly after drawing but then left uncapped, allowing ammonia gas to escape.

Frequently Asked Questions

What does a high ammonia blood test mean?

A high ammonia level (hyperammonemia) means that the liver or urea cycle is failing to clear neurotoxic ammonia from the blood. The most common cause is advanced liver disease or cirrhosis with portosystemic shunting, where blood bypasses the liver. Acute liver failure, Reye syndrome, urea cycle disorders, and certain medications, especially valproic acid, can also raise ammonia. Levels above 100–150 µg/dL are often associated with hepatic encephalopathy, causing confusion, sleepiness, or coma.

Why is ammonia blood test drawn on ice?

Ammonia concentration rises rapidly in a blood tube left at room temperature because red blood cells and plasma enzymes continue metabolic processes that produce ammonia. To obtain an accurate result, the sample must be chilled immediately after venipuncture, usually by placing the tube in an ice-water slurry, and centrifuged within 15–20 minutes. The cold dramatically slows cellular metabolism, preserving ammonia at the venipuncture level. Without strict cold handling, the reported ammonia can be falsely elevated by as much as twofold.

Can valproic acid cause high ammonia without liver damage?

Yes, valproic acid (valproate) routinely elevates ammonia independently of standard liver enzymes. It inhibits the urea cycle enzyme carbamoyl phosphate synthetase I and increases renal glutaminase activity, leading to hyperammonemia even with normal ALT and AST. Some patients on valproate develop ammonia levels exceeding 100 µg/dL and experience lethargy or confusion that resolves when the drug is dose-reduced. This condition, called valproate-induced hyperammonemic encephalopathy, can occur at therapeutic drug levels, so ammonia monitoring is essential during therapy.

How should I prepare for an ammonia blood test?

Fasting for 8–12 hours is ideal, avoiding high-protein meals, intense exercise, and smoking for at least 8 hours before the draw, as these factors transiently raise ammonia. Inform the healthcare provider of all medications, especially valproic acid, carbamazepine, corticosteroids, or recent chemotherapy. The phlebotomist will collect the sample without prolonged tourniquet use and place the tube on ice immediately. In emergency settings where fasting is impossible, the lab will note the non-fasting state and interpret results cautiously. This preparation minimizes false elevations.

What is hepatic encephalopathy and how does ammonia cause it?

Hepatic encephalopathy is a decline in brain function that occurs when the liver cannot remove toxins like ammonia from the blood. Ammonia crosses the blood–brain barrier and is taken up by astrocytes, where it disturbs neurotransmitter balance and causes osmotic swelling. This leads to symptoms ranging from mild confusion and sleep disturbances to seizures and coma. In cirrhosis, portal blood carrying high ammonia bypasses the liver through portosystemic shunts, making ammonia the key measurable toxin. Reducing ammonia with lactulose or rifaximin often reverses the symptoms.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your Ammonia result: