Metabolic
Ammonia
Ammonia is a nitrogen-rich waste product generated mainly by protein breakdown and gut bacteria, and is normally cleared by the liver through the urea cycle. Blood ammonia is most often measured when hepatic encephalopathy or an inborn error of metabolism is suspected, because raised levels can affect the brain.
Why it is measured
Ammonia is highly unstable once blood is drawn, so a near-patient result helps avoid the false elevations caused by transport delays. Point-of-care testing can support faster triage in suspected hepatic encephalopathy and acute metabolic decompensation.
| Typical range | Adults are typically around 11 to 35 micromol/L in plasma, with some laboratories quoting an upper limit closer to 50 micromol/L, and published reference intervals vary more widely between centres. Certain POCT meters report in micrograms/dL, and conversion depends on whether the value is expressed as ammonia or as nitrogen, so figures are not directly comparable across conventions. Ranges vary by method, sample matrix and laboratory: always apply the local reference range and decision limits. |
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| Sample | Whole blood for POCT meters, for example about 20 microlitres for the PocketChem BA, which can use a capillary or venous sample without pretreatment. For laboratory chemistry, free-flowing venous or arterial blood is collected, kept on ice and analysed promptly, because ammonia rises rapidly in vitro. |
| Turnaround | Around 3 minutes on a dedicated point-of-care blood ammonia meter, compared with roughly 30 to 60 minutes plus iced transport for a central laboratory chemistry method. |
Point of care devices that report it
- Arkray PocketChem BA PA-4140 (handheld dry-chemistry blood ammonia meter using a microdiffusion strip method, whole-blood sample of about 20 microlitres)
- Arkray PocketChem BA (earlier model in the same handheld blood ammonia meter line)
Questions, answered
Why does a blood ammonia sample need to be handled so carefully?
Ammonia concentration rises quickly after collection because cells keep releasing it and nitrogen compounds continue to break down. Laboratory samples are kept on ice and analysed without delay, which is one reason a near-patient meter that tests fresh whole blood can be operationally useful. This is general guidance, not a substitute for your local laboratory protocol.
Can a point-of-care ammonia meter replace the laboratory method?
A POCT meter such as the PocketChem BA gives a rapid indicative result and correlates reasonably with laboratory methods, but published evaluations note measurement bias, so significant or unexpected results are usually confirmed by an accredited laboratory. Local policy decides how POCT and laboratory testing are combined and which result is used for clinical decisions.
What can cause a falsely high ammonia result?
Common pre-analytical causes include delayed analysis, a sample left at room temperature, a prolonged tourniquet or difficult venepuncture, smoking near the time of collection and sample contamination. Careful collection and prompt testing reduce these errors. This is educational information only and not advice about any individual result.