Blood ammonia: sample handling and interpretation
Which statement about measuring ammonia is correct?
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A. Ammonia concentration correlates closely with the severity of hepatic encephalopathy
The correlation is poor: some patients have very high ammonia and few signs, while others have obvious encephalopathy with moderate values. Other toxins and inflammation also contribute to encephalopathy.
B. A normal fasting ammonia rules out a portosystemic shunt
Fasting ammonia is fairly sensitive for shunts, but not 100%: a normal value does not rule one out when clinical suspicion is high. Complete the work-up with bile acids and imaging.
C. The sample can be sent chilled to an external laboratory the next day without affecting the result
Ammonia rises in vitro over time through cellular metabolism and amino acid breakdown, even when chilled. Without immediate processing the result is unreliable.
D. The sample must be processed immediately (on ice, with plasma separated within minutes), because ammonia rises in vitro
Correct. Collect into ammonium-free heparin, keep on ice and analyse or separate plasma within minutes. Haemolysis and delay cause false increases, which is why it is usually measured in-clinic.
lightbulb Key takeaway
Ammonia is useful to support hepatic encephalopathy or a shunt, but only if the sample is processed immediately. A normal value does not rule out a shunt and its magnitude does not reflect the severity of signs.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.