What is the critical level of ammonia (ammonia level) in the blood?

Critical Levels of Ammonia in the Blood

The critical level of ammonia in the blood is >150 μmol/L (>255 μg/dL), at which point aggressive intervention should be initiated due to increased risk of neurological deterioration, cerebral edema, and mortality. 1

Normal and Abnormal Ammonia Levels

• Normal range: ≤35 μmol/L (<60 μg/dL) in adults 2
• Hyperammonemia definition:

  • 100 μmol/L (>170 μg/dL) in neonates

  • ≥50 μmol/L (≥85 μg/dL) in term infants, children, and adolescents 2

Risk Stratification Based on Ammonia Levels

Moderate Risk (Requires Monitoring)

• 100-150 μmol/L (170-255 μg/dL): Associated with increased risk of hepatic encephalopathy 3

High Risk (Requires Intervention)

• 150 μmol/L (>255 μg/dL): Threshold for initiating aggressive treatment, including renal replacement therapy 2, 1

• 200 μmol/L (>341 μg/dL): Independent risk factor for mortality, especially in liver failure 4

  • 55% of acute liver failure patients with this level develop intracranial hypertension 3

• 300 μmol/L (>510 μg/dL): Indicates rapid progression requiring immediate CKRT 1

• 400 μmol/L (>680 μg/dL): Severe hyperammonemia requiring immediate intervention regardless of other factors 1

Clinical Decision Algorithm

1. For ammonia >150 μmol/L with neurological deterioration:

   • Initiate Continuous Kidney Replacement Therapy (CKRT) immediately 1
   • Start nitrogen-scavenging agents 1
   • Consider plasma exchange when available 2

2. For ammonia >200 μmol/L regardless of symptoms:

   • Initiate aggressive treatment due to high mortality risk 4
   • Begin hemodialysis for rapid reduction 1
   • Transition to CKRT once levels are <200 μmol/L on two consecutive hourly measurements 1

3. For ammonia >300 μmol/L with rapid rise:

   • Immediate CKRT regardless of symptoms 1
   • Continue nitrogen-scavenging agents during dialysis to prevent rebound 1

4. For ammonia >400 μmol/L with moderate/severe encephalopathy:

   • Emergent CKRT even if refractory to medical management 1

Important Clinical Considerations

• Sample handling is critical: Use EDTA or lithium heparin tube, transport on ice, and process within 15 minutes to avoid falsely elevated results 1, 5

• Variability factors: Ammonia levels can increase by 12-18% after protein meals and vary significantly between testing sites 5

• Monitor for early signs of encephalopathy: Subtle personality changes, mild confusion, sleep disturbances, and decreased attention span 1

• Do not rely solely on ammonia levels for clinical management: Treatment decisions should incorporate clinical status, as ammonia levels alone do not consistently guide therapy in practice 6

• Lack of improvement in ammonia levels by day 5 is associated with 70.6% mortality 7

• Youth, requirement for vasopressors, and need for renal replacement therapy are additional independent risk factors for intracranial hypertension beyond ammonia levels 3

• Temporarily withdraw protein during acute phase of hyperammonemia, but reintroduce within 48 hours after levels decrease to 80-100 μmol/L to avoid catabolism 1

By understanding these critical thresholds and implementing prompt interventions when ammonia levels exceed 150 μmol/L, clinicians can significantly improve outcomes in patients with hyperammonemia.