Finding the right tests…
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Finding the right tests…
Biochemistry
Ammonia is a normal, toxic-if-unchecked byproduct of protein breakdown, made largely by bacteria in your gut, and a healthy liver converts it into urea for the kidneys to clear before it ever builds up. This test measures how much ammonia is currently circulating in your blood, and it's ordered almost exclusively when that conversion process is suspected to be failing, most often in advanced liver disease, where a buildup of ammonia can affect brain function and cause confusion, a condition called hepatic encephalopathy. It's also used in newborns and infants with unexplained vomiting or lethargy, where a rare, inherited fault in the urea cycle itself, rather than liver damage, can be the underlying cause.
The result is reported as a single ammonia concentration from a blood sample, generally read as either within the expected range for age or above it, since normal levels shift somewhat with age. A raised result in someone with known liver disease supports hepatic encephalopathy as the cause of confusion or altered alertness; a markedly raised result in a newborn, especially with no clear liver problem, points instead toward a urea cycle disorder needing urgent further testing. Certain medicines, including some diuretics and opioid pain medicines, can affect the reading, so it's worth telling your doctor everything you're currently taking.
Ammonia is made continuously, mainly by gut bacteria breaking down protein, and a healthy liver clears it just as continuously by converting it into urea, a safer waste product the kidneys pass out in urine. That balance only becomes visible on a blood test when something disrupts it.
When the liver is too damaged to keep up with ammonia conversion, whether from cirrhosis, acute liver failure, or a related condition, ammonia can build up in the blood and cross into the brain, where it's linked to confusion, drowsiness, and in severe cases, coma, the pattern known as hepatic encephalopathy. A raised ammonia level in someone with known liver disease and new confusion supports that diagnosis, though it's read alongside the clinical picture, not as a number that stands alone.
In infants, a markedly high ammonia level with no liver explanation instead raises concern for a urea cycle disorder, a rare inherited condition where one of the enzymes needed to convert ammonia to urea is missing or faulty from birth. Because untreated ammonia buildup can affect the developing brain quickly in this group, a result like this is treated as urgent rather than something to revisit at a routine follow-up. Ammonia moves between tissues mainly as alanine or glutamine, its less toxic carrier forms, before the liver converts the freed ammonia into urea.
It supports hepatic encephalopathy, confusion or altered alertness caused by ammonia building up and affecting the brain, as the reason for new or worsening confusion.
A markedly high ammonia level in an infant with no clear liver cause can point toward a rare inherited urea cycle disorder, which needs urgent further testing.
Yes. Some diuretics and opioid pain medicines are known to affect ammonia readings, so it helps to tell your doctor about everything you're currently taking.
No. While liver disease is the most common adult cause, other causes exist, including certain inherited conditions, especially in infants, so the full clinical picture matters.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
When the liver can't clear ammonia properly, it builds up in the blood and can cross into the brain, where it's linked to confusion, drowsiness, and in severe cases, coma.
It's a blood test. Handling the sample correctly matters a great deal for this particular test, which is why a related catalogue entry specifies plasma rather than a routine blood tube.