Ammonia Interpretation Calculator
Ammonia Interpretation Calculator
Grade plasma ammonia by age group and severity, and see why sampling technique matters as much as the number itself.
Ammonia Interpretation
NH3 → urgency by agePlasma ammonia 145 µmol/L, adult
Thresholds and why they differ by age
Neonate: <50 normal · 50–99 possibly normal for age · 100–150 raised · >150 above the treatment line
- neonatal treatment line
- above 150 µmol/L in a neonate and above 100 µmol/L outside the neonatal period, per the 2019 urea cycle disorder guideline (Table 4, symptomatic patients)
- neonatal urgency
- a urea cycle disorder must be excluded within hours in a hyperammonaemic neonate; treatment starts before the diagnosis is confirmed
- pre-analytical error
- a tourniquet, clenched fist, delayed transport or delayed separation all raise the measured value spuriously
- hepatic encephalopathy
- ammonia correlates poorly with grade in chronic liver disease and should not be used to diagnose or grade it
Worked example
Plasma ammonia 145 µmol/L, adult
Older than 28 days: 145 is below 200 but at or above 100 → Raised
Moderate hyperammonaemia — investigate promptly
Ammonia thresholds by age
| Ammonia (µmol/L) | Older than 28 days | Neonate (first 28 days) |
|---|---|---|
| < 50 | Normal | Normal |
| 50 – 99 | Mildly raised | Possibly normal for age — neonatal upper limit about 82 |
| 100 – 150 | Raised — above the treatment line | Raised — stop protein, IV glucose, repeat urgently |
| 151 – 199 | Raised | Above the neonatal treatment line — start drug treatment |
| ≥ 200 | Markedly raised | Above the treatment line; from 250 prepare haemo(dia)filtration |
Reading ammonia by age, and by sample quality
In a neonate, hyperammonaemia is a medical emergency. A urea cycle disorder must be excluded within hours, because irreversible neurological damage relates to both the peak ammonia level and the duration of exposure — not to the eventual diagnosis. Treatment therefore begins before the underlying cause is confirmed. The 2019 urea cycle disorder guideline sets the line for starting drug treatment in a symptomatic neonate above 150 µmol/L, and outside the neonatal period above 100 µmol/L; below those, above the normal limit, it advises stopping protein, giving IV glucose and repeating urgently. The neonatal reference interval is itself higher than the adult one, which is why a neonatal value in the 50s to 90s is read here as possibly normal for age rather than raised. The neonatal hyperammonaemia interpreter covers the neonatal picture in detail.
In adults, the commonest causes are liver failure and portosystemic shunting, followed by valproate therapy, and — rarely — a late-presenting urea cycle disorder unmasked by pregnancy, surgery, gastrointestinal bleeding or an unusually high protein load. The response is less urgent than in a neonate, but the same principle of excluding a treatable cause applies.
The pre-analytical warning bears repeating prominently, because it changes management in practice. A tourniquet, a clenched fist during sampling, delayed transport, or a delay in separating the plasma all raise the measured ammonia — and a spuriously high result that triggers unnecessary escalation is a real and avoidable harm. Any unexpectedly raised value on a poorly handled sample should be repeated with proper technique before it is acted on.
Ammonia should not be used to diagnose or grade hepatic encephalopathy in chronic liver disease. It correlates poorly with clinical severity — patients with normal ammonia can be deeply encephalopathic, and patients with markedly raised levels can be lucid — so guidelines are consistent that the diagnosis and grading of encephalopathy remain clinical, made at the bedside rather than from this number.
Frequently asked questions
Why is hyperammonaemia more urgent in a neonate?
A urea cycle disorder must be excluded within hours, because neurological damage depends on both the peak level and the duration of exposure. Treatment starts before the diagnosis is confirmed.
At what ammonia level is treatment started in a neonate?
The 2019 urea cycle disorder guideline starts drug treatment — IV L-arginine and sodium benzoate, alongside stopping protein and giving IV glucose — above 150 µmol/L in a symptomatic neonate, and above 100 µmol/L outside the neonatal period. From 250 µmol/L it advises preparing haemo(dia)filtration, and from 500 µmol/L starting it immediately. Its 80–150 µmol/L re-assay band is for the prospective care of an at-risk newborn with an affected sibling.
What are the commonest causes of raised ammonia in adults?
Liver failure and portosystemic shunting, followed by valproate therapy. A late-presenting urea cycle disorder is rare but can be unmasked by pregnancy, surgery or a high protein load.
Why does a tourniquet affect the ammonia result?
Muscle releases ammonia during venous stasis and fist clenching, producing a spuriously high result. Sampling technique and rapid processing matter as much as the underlying disease.
Should ammonia be used to grade hepatic encephalopathy?
No. It correlates poorly with clinical severity in chronic liver disease. Encephalopathy should be diagnosed and graded clinically, not by the ammonia level.
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References
- Häberle J, Burlina A, Chakrapani A, et al. Suggested guidelines for the diagnosis and management of urea cycle disorders: first revision. J Inherit Metab Dis. 2019;42(6):1192–1230. doi:10.1002/jimd.12100. Table 4, levels of hyperammonaemia and suggested actions in symptomatic patients: drug treatment above 100 µmol/L, with the footnote "This limit of action applies for patients outside the neonatal period; for neonates use >150 and <250"; haemo(dia)filtration prepared at 250–500 µmol/L and started immediately at 500–1000.
- Häberle J, Boddaert N, Burlina A, et al. Suspected urea cycle disorders: European guidelines. Orphanet J Rare Dis. 2012;7:32. The first edition, superseded by the 2019 revision above.
- Diaz J, et al. Proposed plasma ammonia reference intervals in a reference group of hospitalized term and preterm neonates. J Appl Lab Med. 2020;5(2):363–372. Upper reference limit 82 µmol/L in neonates under one week; one cohort on one platform.
- Vilstrup H et al. Hepatic encephalopathy in chronic liver disease: 2014 practice guideline by AASLD and EASL. Hepatology. 2014;60(2):715–735.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/