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/infant: <50 normal · 50–99 mildly raised · 100–199 markedly raised · ≥200 critical
- 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
Adult pathway: 145 is below 200 but at or above 100 → Raised
Moderate hyperammonaemia — investigate promptly
Ammonia thresholds by age
| Ammonia (µmol/L) | Adult | Neonate/infant |
|---|---|---|
| < 50 | Normal | Normal |
| 50 – 99 | Mildly raised | Mildly raised |
| 100 – 199 | Raised | Markedly raised — urgent |
| ≥ 200 | Markedly raised | Critical — metabolic emergency |
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: stopping protein intake, providing calories to halt catabolism, starting ammonia-scavenging drugs, and arranging haemofiltration for the most severe cases.
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 threshold for concern is lower and the response 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.
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, Boddaert N, Burlina A, et al. Suspected urea cycle disorders: European guidelines. Orphanet J Rare Dis. 2012;7:32.
- Vilstrup H et al. Hepatic encephalopathy in chronic liver disease: 2014 practice guideline by AASLD and EASL. Hepatology. 2014;60(2):715–735.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.