Normal Serum Ammonia Values
In healthy adults, normal blood ammonia concentrations are ≤35 μmol/L (≤60 μg/dL). 1
Age-Specific Reference Ranges
The normal ammonia level varies significantly by age, and using the wrong reference range can lead to misdiagnosis:
- Neonates (1-7 days old): ≤110 μmol/L (≤154 μg/dL) 1
- Infants (8-14 days old): <90 μmol/L (≤126 μg/dL) 1
- 15 days to adulthood: 16-53 μmol/L (22-74 μg/dL) 1
- Adults: ≤35 μmol/L (≤60 μg/dL) 1
Some sources report a slightly broader adult range of 11-50 μmol/L during normal physiology. 2
Clinical Thresholds for Hyperammonemia
Understanding when ammonia becomes pathological is critical for timely intervention:
- Neonates: Hyperammonemia is defined as >100 μmol/L (170 μg/dL) 1, 3
- Term infants, children, and adults: Hyperammonemia is defined as ≥50 μmol/L (85 μg/dL) 1, 3
- Levels >200 μmol/L (341 μg/dL): Associated with poor neurological outcomes 1
- Severe hyperammonemia (>400 μmol/L or 681 μg/dL): May require kidney replacement therapy 1
Critical Sample Collection Requirements
Improper sample handling is the most common cause of falsely elevated ammonia results. 1 To obtain accurate measurements:
- Collect blood from a free-flowing venous or arterial sample without tourniquet or fist clenching 1, 4
- Use lithium heparin or EDTA tubes 1, 4
- Transport on ice immediately to the laboratory 1, 4
- Process within 15 minutes of collection and analyze immediately 1, 3
- Collect from fasting patients when possible 1
- Clean skin thoroughly before collection 1
- Fill tube completely, secure lid, and homogenize by inversion 1
If immediate processing is impossible, centrifuge at +4°C and freeze the supernatant at −70/−80°C. 1
Common Pitfalls That Falsely Elevate Results
Several factors can produce artifactually high ammonia levels:
- Sample hemolysis releases ammonia from red blood cells 1
- Delayed processing allows continued ammonia production in the sample 1, 5
- Venous stasis from prolonged tourniquet use 1, 4
- High lipemia and jaundice interfere with measurement 1
- High-protein diet, smoking, exercise, and certain drugs (especially valproic acid) can elevate levels 1, 4
Clinical Context and Interpretation
A normal ammonia value (<35 μmol/L in adults) has high negative predictive value and should prompt reconsideration of hepatic encephalopathy as a diagnosis. 4 Conversely, ammonia levels alone should not confirm hepatic encephalopathy, as cirrhotic patients may have elevated levels without encephalopathy. 1 However, when encephalopathy is present, ammonia levels do correlate with severity and prognosis. 1
Elevated ammonia >100 μmol/L with a family history of liver disease, neurological disorders, or severe neurological impairment should prompt investigation for inherited metabolic disorders such as urea cycle defects. 1, 4