Unit Converter
Ammonia (NH3)
(Plasma/Serum Ammonia – Hepatic Encephalopathy Marker)
Synonyms
- Ammonia
- NH₃
- Blood ammonia
- Plasma ammonia
- Venous ammonia
- Arterial ammonia (critical care)
Units of Measurement
µmol/L, µg/dL, µg/100mL, µg%, µg/mL, µg/L, mg/L
Description
Ammonia (NH₃) is a toxic nitrogenous compound produced from:
- Amino acid metabolism
- Bacterial activity in the intestine
- Renal ammoniagenesis
- Muscle protein breakdown
Normally, the liver converts ammonia → urea via the urea cycle.
When liver function is impaired or blood flow bypasses the liver, ammonia accumulates, leading to hepatic encephalopathy (HE).
Ammonia measurement is essential for evaluating:
- Hepatic encephalopathy
- Acute liver failure
- Urea cycle disorders
- Reye syndrome
- Severe neonatal illness
Physiological Role
- Nitrogen transport intermediate
- Acid–base balance (kidney generates NH₄⁺ to buffer acid load)
- Brain neurotransmission (excess ammonia disrupts astrocytes → cerebral edema)
Clinical Significance
Elevated Ammonia (Hyperammonemia)
1. Liver-Related Causes
- Acute liver failure
- Cirrhosis with portosystemic shunting
- Fulminant viral hepatitis
- Alcoholic hepatitis
2. Urea Cycle Disorders (UCD)
- Ornithine transcarbamylase deficiency
- CPS-I deficiency
- ASS/ASL deficiency
(Severe elevations → neonatal coma)
3. Medications / Toxins
- Valproate toxicity
- Chemotherapy
- Carbamazepine
- Salicylates
- Toxins causing mitochondrial dysfunction
4. Other Causes
- GI bleeding (↑ protein load)
- Severe infection/sepsis
- Reye syndrome
- Gastric bypass / portosystemic shunts
- Total parenteral nutrition
- Renal failure (mild–moderate)
Ammonia > 150–200 µmol/L → risk of cerebral edema, especially in acute liver failure.
Low Ammonia
Not clinically significant; occurs rarely with:
- Overheparinized sample
- Excessive handling delays
- Hypothermia
Reference Intervals
(Tietz 8E + Mayo + ARUP + AASLD)
Adults (Venous Plasma)
- 10 – 50 µmol/L
- ≈ 17 – 85 µg/dL
Arterial Plasma
- Lower values: ~15–45 µmol/L
(more accurate in hepatic encephalopathy)
Newborns
- < 100 µmol/L normal
- >150–200 µmol/L → strongly suggests UCD
Sample Collection – CRITICAL
Ammonia is unstable. Errors are common.
To ensure accuracy:
- Collect in pre-chilled EDTA or heparin tube
- Place on ice immediately
- Centrifuge within 15–30 minutes
- Avoid use of a tourniquet
- Avoid fist clenching
- Avoid hemolysis
- Analyze promptly
Delayed processing → false elevation
Unit Meanings
| Unit | Meaning |
| µmol/L | micromole per liter |
| µg/dL | microgram per deciliter |
| µg/100 mL | same as µg/dL |
| µg% | microgram per 100 mL |
| µg/mL | microgram per milliliter |
| µg/L | microgram per liter |
| mg/L | milligram per liter |
Diagnostic Uses
1. Hepatic Encephalopathy
- Elevated ammonia supports diagnosis
- NOT a stand-alone diagnostic tool
- Arterial ammonia correlates better with HE grade
2. Acute Liver Failure
High ammonia → indicates:
- Cerebral edema risk
- Need for ICU management
- Predicts poor prognosis
3. Inborn Errors of Metabolism
Markedly high ammonia in neonates suggests:
- Urea cycle defects
- Organic acidemias (propionic, methylmalonic)
- Fatty acid oxidation disorders
4. Monitoring Therapy
Used to monitor response to:
- Lactulose
- Rifaximin
- Hemodialysis (in severe hyperammonemia)
Analytical Notes
- Venous vs arterial sample differences must be noted.
- Capillary samples inaccurate for ammonia.
- Protein-rich meals may increase ammonia slightly.
- Plasma preferred over serum.
Clinical Pearls
- Ammonia correlates poorly with mental status in chronic HE.
- Extremely high ammonia (>200–300 µmol/L) → consider UCD, not cirrhosis.
- GI bleeding increases ammonia due to protein breakdown.
- Valproate inhibits carbamoyl phosphate synthetase → hyperammonemia without liver failure.
- Ammonia rises quickly after sample mishandling — a major cause of false positives.
Interesting Fact
Ammonia was first recognized as a cause of hepatic coma in the 1950s. Despite limitations, it remains an essential biomarker in acute liver failure and inborn metabolic disorders.
References
- Tietz Clinical Chemistry and Molecular Diagnostics, 8th Edition - Ammonia Testing.
- Mayo Clinic Laboratories - Ammonia Test Interpretation.
- ARUP Consult - Hyperammonemia Diagnostic Pathway.
- AASLD - Hepatic Encephalopathy Guidelines.
- IFCC - Analytical Recommendations for Ammonia.
- NIH / MedlinePlus - Ammonia Overview.
- Emergency Medicine & Hepatology Reviews - Ammonia in Critical Care.