Cholinesterase Test
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. To investigate the cause of temporary paralysis following treatment with suxamethonium during surgery.
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; 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 which in turn can lead to overstimulation of nerves in the body tissues.
Serum butyrylcholinesterase is the enzyme measured by the laboratory because it is this enzyme which is involved with the metabolism of the muscle relaxant suxamethonium used in surgery and which is also affected by exposure to organic phosphorus insecticides.
The laboratory measures the total activity of cholinesterase in the patient’s blood serum. This is useful because if the level 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 levels in people working with organophosphorus compounds on farms or in the chemical industry 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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
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 reacting muscle relaxant suxamethonium or mivacurium.
- 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 in 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. Individuals with a usual UU phenotype have no risk whereas UA or UF phenotypes may carry a small risk of an increased recovery time of about half an hour if suxamethonium is used during pregnancy, say during a caesarean section. Individuals with AA or AS phenotypes can experience prolonged paralysis of 2 or more hours with the SS phenotype over three hours. The other fluoride phenotypes FF, FS and AF tend to experience an intermediate sensitivity to suxamethonium.
- 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
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
