What this test measures
Ammonia (NH3) is a nitrogen-containing waste product of protein metabolism, produced largely by gut bacteria and amino acid breakdown. The liver clears it by converting ammonia to urea via the urea cycle; urea is then excreted by the kidneys. When the liver fails or the urea cycle is impaired, ammonia accumulates and crosses the blood-brain barrier, causing confusion, asterixis (flapping tremor), seizures and coma.
Ammonia must be measured in a tube placed on ice immediately after collection and processed within 30 minutes — otherwise red cells and proteins continue to generate ammonia and the result is falsely high. Most labs require an arterial or venous sample drawn without a tourniquet (or minimal tourniquet) and free-flowing.
Why it matters
In India, hepatic encephalopathy is common in patients with cirrhosis (hepatitis B/C, alcoholic, NAFLD-related) and in acute liver failure. While diagnosis is primarily clinical (West Haven grading), ammonia helps when the picture is unclear — for example, in a confused cirrhotic patient where overlap with infection, alcohol withdrawal or metabolic causes makes interpretation difficult.
Ammonia is also essential in neonates and children with unexplained vomiting, lethargy, coma or unexplained metabolic decompensation — urea cycle disorders and organic acidaemias are treatable if recognised early. In adults, it can flag valproate toxicity, urinary tract infection with urea-splitting organisms (sometimes), and rare urea cycle disorder presentations.
How to prepare
Strict sample handling matters more than patient preparation. No fasting required, but ideally avoid food for 4–6 hours and avoid smoking 6 hours before (smoking raises ammonia). Avoid heavy exercise. Sample drawn into a pre-cooled lithium heparin or EDTA tube, placed immediately on ice, transported to the lab within 30 minutes — confirm with the lab in advance.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Ammonia (µmol/L)[1][2] | Adults 11 – 35 · Children 21 – 50 · Newborns up to 90 | Low ammonia is rarely clinically significant. | Mild rise (35–80) — non-specific; can be drug-related (valproate, glycine), high-protein meal, sample handling artefact, urinary tract infection with urea-splitters, mild liver disease. Moderate rise (80–150) — chronic liver disease with encephalopathy, GI bleed in a cirrhotic, valproate toxicity. Marked rise (>150) — acute liver failure with encephalopathy, urea cycle disorders (especially in children/neonates — needs urgent metabolic workup), Reye syndrome, organic acidaemias. Values > 200 in adults are usually associated with significant neurological compromise. |
When to suspect hyperammonaemia
| Setting | Likely cause | Action |
|---|---|---|
| Confused adult with cirrhosis | Hepatic encephalopathy (often triggered by infection, GI bleed, constipation, drugs) | Lactulose, rifaximin, identify trigger; serial neuro exam |
| Acute liver failure (any age) | Ammonia >150 predicts cerebral oedema risk | ICU care, possible transplant evaluation |
| Neonate / child with vomiting, lethargy | Urea cycle disorder, organic acidaemia, fatty acid oxidation defect | Urgent metabolic panel + paediatric metabolic referral |
| Patient on valproate with new confusion | Valproate-induced hyperammonaemia (can occur with normal liver enzymes) | Stop / reduce valproate; check with neurologist |
| Postoperative or post-bariatric surgery | Acquired urea cycle stress (rare) | Stop protein loading; nephrology / metabolic consult |
Frequently asked questions
Why does the ammonia sample need ice?
Red cells and proteins keep generating ammonia after the sample is drawn — at room temperature, levels rise by 20% in 30 minutes. Ice plus immediate transport keeps the result accurate.
Do I need to fast?
Not strictly. But avoid food for 4–6 hours and avoid smoking for 6 hours — both raise ammonia.
Is a high ammonia always due to liver disease?
No. The commonest cause in adults is liver disease (cirrhosis, acute liver failure), but other causes include valproate toxicity, urea cycle disorders, urinary tract infections with urea-splitting organisms (Proteus), some intestinal bypass surgeries, and high-protein loads.
Why is ammonia important in children?
Urea cycle disorders, organic acidaemias and fatty acid oxidation defects can present in neonates and infants with vomiting, lethargy or coma. Ammonia is the single most important test in unexplained neonatal encephalopathy — values >150 µmol/L need immediate metabolic and dialysis evaluation.
Can ammonia be normal in hepatic encephalopathy?
Yes. Ammonia correlates only loosely with encephalopathy grade. The diagnosis is clinical (confusion, asterixis, sleep changes in a known cirrhotic). Ammonia helps when the picture is unclear but a normal value does not rule it out.
How is hepatic encephalopathy treated?
Lactulose (acidifies gut, traps ammonia as ammonium) and rifaximin (poorly-absorbed antibiotic that reduces ammonia-producing gut bacteria) are the mainstays. Triggers (infection, GI bleed, constipation, sedatives, electrolyte imbalance) must be identified and corrected.
Can valproate cause high ammonia?
Yes — valproate-induced hyperammonaemic encephalopathy can occur even with normal liver enzymes and is reversible on stopping the drug. Always consider it in a patient on valproate who develops new confusion.
Related Liver / Enzymes tests
Tests commonly ordered alongside AMMONIA, or that help interpret an unexpected result.
Sources & references
- AASLD Practice Guidance — Hepatic Encephalopathy · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Hyperammonemia · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Ammonia Test · accessed 2026-05-30T00:00:00.000Z
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