Ammonia Levels are helpful to support the diagnosis of severe liver diseases (fulminant hepatitis or cirrhosis), and for surveillance of these diseases. Ammonia levels are also used in the diagnosis and follow-up of hepatic encephalopathy.
Ammonia is a by-product of protein catabolism. Most of it is made by bacteria acting on proteins present in the gut. By way of the portal vein, it goes to the liver, where it is normally converted into urea and then secreted by the kidneys. Ammonia cannot be catabolized in the presence of severe hepatocellular dysfunction. Furthermore, when portal blood flow to the liver is altered (e.g., in portal hypertension), ammonia cannot reach the liver to be catabolized. Ammonia blood levels rise. Congenital enzymatic defects in the urea cycle also can cause a rise in ammonia levels. Finally, impaired renal function diminishes excretion of ammonia, and the blood levels rise. High levels of ammonia result in encephalopathy and coma. Arterial ammonia levels are more reliable than venous levels but more difficult to obtain and are therefore not routinely used.
Normal Ammonia Levels
Ammonia is measured in terms of micrograms per deciliter (mcg/dL). Normal Ammonia Levels are the highest among newborns. Normal Ammonia Levels decrease by aging, The following is a listing of Normal Ammonia Levels:
Newborn: Between 90 and 150 mcg/dL.
Child: Between 40 and 80 mcg/dL.
Adult: Between 10 and 80 mcg/dL.
Causes of Ammonia Levels False Indications
- Hemolysis increases ammonia levels because the red blood cells (RBCs) have about three times the ammonia level content of plasma.
- Muscular exertion can increase ammonia levels.
- Cigarette smoking can produce significant increases in ammonia levels within 1 hour of inhalation.
- Ammonia levels may be factitiously increased if the tourniquet is too tight for too long.
- Drugs that may cause increased ammonia levels include Acetazolamide, Alcohol, Ammonium Chloride, Barbiturates, Diuretics (Loop, Thiazide), Narcotics and Parenteral Nutrition.
- Drugs that may cause decreased levels include broad-spectrum Antibiotics (e.g., Neomycin), Lactobacillus, Lactulose, Levodopa, and Potassium salts.
Indications of High Ammonia Levels
- Abnormally high Ammonia Levels can be an indication of Primary Hepatocellular Disease, Reye Syndrome, or Asparagine Intoxication: since in these conditions there are not enough functioning liver cells to metabolize the ammonia.
- High levels of Ammonia are associated with Portal Hypertension, and Severe Heart Failure With congestive Hepatomegaly because the portal blood flow from the gut to the liver is altered and the ammonia cannot get to the liver to be metabolized for excretion. Furthermore, the ammonia from the gut is rapidly shunted around the liver (by way of gastroesophageal varices) and into the systemic circulation.
- Hemolytic Disease of newborn (Erythroblastosis Fetalis): RBCs contain high amounts of ammonia. The newborn liver is not mature enough to metabolize all the ammonia presented to it by the hemolysis that occurs in this disease.
- High Ammonia Levels can be an indication of Gastrointestinal Bleeding with mild liver disease, or Gastrointestinal Obstruction with mild liver disease since Ammonia production is increased because the bacteria have more protein (blood) to catabolize. An impaired liver may not be able to keep up with the increased load of ammonia presented to it.
- Hepatic Encephalopathy and Hepatic Coma: These neurologic states are a result of ammonia acting as false neurotransmitters. The brain cannot function properly.
- Genetic Metabolic Disorder of Urea Cycle: Ammonia is catabolized by the urea cycle. Disruption of that cycle will inhibit excretion of ammonia and levels can be expected to rise.
Causes of Low Ammonia Levels
- Essential or Malignant Hypertension.