Cholinesterase
At a Glance
Why Get Tested?
To find out if you are likely to have temporary paralysis after being given a muscle relaxant called suxamethonium during surgery. This temporary paralysis is called suxamethonium apnoea.
Cholinesterase can also be used to screen for exposure to the effects of organic phosphorus insecticides.
When to Get Tested?
If you or a close relative have experienced suxamethonium apnoea after a surgical operation.
To indicate possible insecticide poisoning with recent or frequent use of organic phosphorus insecticides. Occupational exposure can occur in workers involved with agriculture or the organic chemical industry.
Sample Required?
A blood sample taken from a vein in your arm
Test Preparation Needed?
None.
The Test Sample
What is being tested?
There are two similar cholinesterase enzymes in the body; butyrylcholinesterase (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 whether butyrylcholinesterase levels are low and can quickly find out if the patient is at risk. Low levels can be inherited or acquired as described above.
If the butyrylcholinesterase activity is low the then phenotype studies may be used to help confirm the presence of an unusual (atypical) 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 inhibitors such as dibucaine and fluoride. The percentage of the enzyme activity remaining is referred to as a ‘number’ e.g. ‘dibucaine number’, ‘fluoride 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.
Genetic studies can also be necessary as second-line tests if an atypical variant is difficult for the laboratory to identify or if a silent S gene is suspected. Some laboratories perform these instead of phenotype studies. However in Australia there is currently no Medicare rebate for cholinesterase genetic studies.
How is the sample collected for testing?
A blood sample drawn from a vein in your arm
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
- Cholinesterase levels and phenotype studies in a patient can be used to predict the degree of potential post-operative paralysis following the use of the short acting muscle relaxant suxamethonium.
- Cholinesterase can be used as a monitor of exposure to organic phosphorus compounds in the agricultural and chemical industries.
When is it requested?
- Pre-operative screening for cholinesterase activity and phenotype is advised if the patient or a close family member has experienced prolonged paralysis after an operation that required ventilation support up to several hours
- People using organic phosphorus compounds such as Parathion, Sarin or tetraethyl pyrophosphate in the farming or chemical industries may be routinely monitored to assess any adverse exposure once baseline levels are established. Cholinesterase levels can also be used to assess any acute exposure to these compounds which can cause neuromuscular damage. Toxicity can follow a rapid absorption of the compound through the lungs, skin or gastrointestinal tract. The symptoms of toxicity are varied, ranging from vomiting to paralysis or coma, and depend on the compound, quantity and the site of exposure.
What does the test result mean?
The degree of sensitivity to suxamethonium varies according to the patient’s phenotype.
| Phenotype | Risk of paralysis after suxamethonium |
| UU | None |
| UA | Minor (may occur in pregnant women) |
| UF | Minor (may occur in pregnant women) |
| FF | Moderate |
| FS | Moderate |
| AF | Moderate |
| AA | High |
| AS | High |
| SS | High |
Following exposure to organic phosphorus compounds, serum cholinesterase can fall to about 40% before any symptoms occur and an 80% drop before the symptoms become severe.
Is there anything else I should know?
In addition to cholinesterase variants and poisoning with organophosphates, total cholinesterase activity can be lowered in a number of other conditions. These include pregnancy, renal disease, shock, malnutrition and some cancers.
As cholinesterase is synthesised by the liver the activity can also be lower in some liver diseases such as acute and chronic hepatitis, advanced cirrhosis and liver metastases. However, normal levels can be found in mild hepatitis and cirrhosis as well as obstructive jaundice.
There may be a risk of a very mild prolonged reaction to suxamethonium in these conditions of minutes as opposed to hours, as long as these patients have lower activities of the usual enzyme rather than the atypical enzyme variant.
Common Questions
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Article Sources
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