What is being tested?There are two similar cholinesterase enzymes in the body; butrylcholinesterase (pseudocholinesterase) found in the blood plasma, intestine and white matter, and Acetylcholinesterase found in red blood cells, grey matter and nerve endings.
Since butyrylcholinesterase is the enzyme which is involved with the metabolism of the muscle relaxant suxamethonium and affected by exposure to organic phosphorus insecticides, it is this enzyme which is measured in these situations.
The laboratory measures the total activity of cholinesterase in the patient’s blood plasma. This is useful because it indicates if an atypical enzyme variant (phenotype) is present and can quickly find out if the patient is at risk.
Phenotype studies are used to help confirm an usual enzyme, 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 determine whether the patient has a usual (normal) or an atypical enzyme. The enzyme phenotype can be further classified as an F or K variant.
Genetic studies can also be necessary 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.
