There are two similar cholinesterase in the body; butrylcholinesterase (pseudocholinesterase) found in the blood plasma, intestine and white matter of the brain, and acetylcholinesterase found in red blood cells, grey matter of the brain and nerve endings.
Acetylcholine is a chemical which is involved in the transmission of signals across nerve endings and which is broken down by the enzyme acetylcholinesterase. Any decrease in this enzyme can cause a build-up of acetylcholine at nerve endings that in turn can lead to overstimulation of nerves in the body tissues.
Serum butyrylcholinesterase is the enzyme usually measured by the laboratory. This is because the enzyme can be used as a marker for the ineffective metabolism of the muscle relaxant suxamethonium used in surgery as well as exposure to organic phosphorus insecticides.
The laboratory measures the total activity of cholinesterase in the patient’s blood. This is useful because if the activity is low it suggests an atypical enzyme variant is present and that the patient is at risk of prolonged effects from the muscle relaxants suxamethonium or mivacurium. Also, low total enzyme activity in people working with organophosphorus compounds on farms or in the chemical industry may indicate acute exposure.
Inhibitor studies are also carried out to determine the patient’s phenotype which can be used to find out whether the enzyme is ‘usual’ or ‘atypical’ , provide more information about any potential risk and assist in family studies. In order to identify the cholinesterase phenotype the enzyme is incubated with a range of inhibitors which include dibucaine, fluoride and frequently Ro 02-0683, chloride and sometimes scoline. The percentage of the enzyme activity remaining is referred to as a ‘number’ and is used to assess the phenotype. In this way the laboratory can discover whether the patient has a usual (normal) or an atypical enzyme. The enzyme phenotype can be further classified into a number of possible variants with varying degrees of possible sensitivity to suxamethonium or mivacurium.
Genetic studies can also be indicated if an ‘atypical’ variant is difficult for the laboratory to identify or if a silent S gene is suspected. These tests are not as widely available as phenotype studies at present.
How is the sample collected for testing?
A blood sample taken from a vein in your arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.