What are normal blood ammonia levels?

Normal Blood Ammonia Levels

Normal blood ammonia concentrations are ≤35 μmol/L (≤60 μg/dL) in adults, with age-specific reference ranges for pediatric populations. 1

Age-Specific Normal Ranges

The normal ammonia levels vary significantly by age, requiring careful interpretation based on the patient's developmental stage:

• 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

During normal physiology, blood ammonia levels typically range between 11 and 50 μmol/L across most age groups. 2

Clinical Thresholds for Hyperammonemia

Understanding when ammonia becomes pathologically elevated is critical for clinical decision-making:

• 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
• Threshold for poor neurological outcomes: Levels >200 μmol/L (341 μg/dL) are associated with significant neurological damage 1, 4, 5
• Severe hyperammonemia requiring dialysis: >400 μmol/L (681 μg/dL) in pediatric patients 1

The threshold of ≥200 μmol/L within the first 48 hours represents an independent risk factor for mortality, with an odds ratio of 3.3 for death. 5

Critical Sampling Requirements

Accurate ammonia measurement is highly dependent on proper collection and handling techniques, as improper technique can lead to falsely elevated results:

Collection Protocol

• Draw from free-flowing venous or arterial blood without venous stasis (no tourniquet use or fist clenching) 1
• Use lithium heparin or EDTA tubes for sample collection 1
• Fill tube completely and secure lid, then homogenize by inversion 1
• Clean skin thoroughly before collection to avoid contamination 1
• Collect from fasting patients when possible 1

Processing Requirements

• Transport sample on ice immediately to the laboratory 1
• Process within 15 minutes of collection and analyze immediately 1, 6
• If delayed measurement is necessary, centrifuge at +4°C and freeze the supernatant at −70/−80°C 1

Common Sources of Error

• Sample hemolysis can falsely elevate ammonia results 1
• Delayed processing leads to falsely elevated levels due to continued ammonia production in the sample 1, 6
• High lipemia and jaundice can interfere with ammonia measurement 1
• High-protein diet, smoking, exercise, and certain drugs can affect ammonia levels 1

Clinical Context and Interpretation

A normal ammonia value casts doubt on the diagnosis of hepatic encephalopathy, though ammonia levels alone should not be used to confirm this diagnosis. 1

• Ammonia levels may be elevated in cirrhotic patients without encephalopathy 1
• When elevated, ammonia levels correlate with the severity/grade of hepatic encephalopathy and prognosis 1
• Elevated ammonia levels >100 μmol/L with family history of liver disease, neurological disorders, or severe neurological impairment should prompt investigation for inherited metabolic disorders 1

The measurement of ammonia remains fraught with technical challenges, and standardization with rigid adherence to collection and processing techniques is essential for reliable test results. 6