What is being tested?
Cholinesterases are enzymes that are involved in helping the nervous system to function properly. There are two separate cholinesterase enzymes in the body: (1) acetylcholinesterase, found in red blood cells as well as in the lungs, spleen, nerve endings, and the gray matter of the brain, and (2) pseudocholinesterase (butyrylcholinesterase), found in the serum as well as the liver, muscle, pancreas, heart, and white matter of the brain. Cholinesterase tests measure the activity of these enzymes.
Acetylcholinesterase is involved in transmission of nerve impulses by breaking down acetylcholine, a chemical that helps to transmit signals across nerve endings. A decrease in the activity of the enzyme acetylcholinesterase results in excess acetylcholine at nerve endings. This can lead to overstimulation of nerves within body tissues and organs. Pseudochlinesterase is involved in processing and metabolizing drugs.
The two most common reasons for testing activity levels in the blood are:
- Organophosphate pesticide exposure. Insecticides containing organophosphates can inhibit cholinesterase and pseudocholinesterase activity. Symptoms can be severe with acute exposure to these pesticides or can gradually appear with chronic exposure. Absorption can occur by inhalation, ingestion, or contact with the skin. Testing red blood cell acetylcholinesterase and serum pseudocholinesterase may be done to detect acute poisoning or to monitor those with occupational exposure to these chemicals, such as farm workers or those who work with industrial chemicals.
- Inherited pseudocholinesterase deficiency. Some individuals have an inherited deficiency due to a genetic variant of the enzyme pseudocholinesterase. This enzyme is used by the body to inactivate (metabolize) succinylcholine, a muscle relaxant that is commonly used during surgery. People who have low levels or defective pseudocholinesterase may experience prolonged effects of the drug, with protracted muscle paralysis and apnea following anesthesia. In addition, those who are homozygous for genetic variants may be at greater risk of adverse effects than those who are heterozygous. Pseudocholinesterase testing can be performed prior to surgery on those with a family history of prolonged apnea after use of succinylcholine to determine if they are at risk of complications related to this drug.
How is it used?
Cholinesterase testing has two main uses:
- It can be used to detect and diagnose organophosphate pesticide exposure and/or poisoning. It may also be used to monitor those who may be at increased risk of exposure to organophosphate compounds, such as those who work in agricultural and chemical industries, and to monitor those who are being treated for exposure. Typically, tests for red blood cell acetylcholinesterase (AChE) and serum pseudocholinesterase (PChE) are used for this purpose.
- It can be used several days prior to a surgical procedure to determine if someone with a history of or family history of post-operative paralysis following the use of succinylcholine, a common muscle relaxant used for anesthesia, is at risk of having this reaction. In these cases, the test for pseudocholinesterase is usually used. A second test, referred to as a dibucaine inhibition test, may be done to help determine the extent to which the activity of the enzyme is decreased.
When is it ordered?
People who work with organophosphate compounds in the farming or chemical industries may be routinely monitored to assess any adverse exposure, once baseline levels have been established. Cholinesterase testing 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 depending on the compound, quantity, and the site of exposure. Early symptoms may include:
- Headache, dizziness
- Excessive tearing in the eyes, sweating and/or salivation
As the effects of the poisoning worsen, some additional symptoms may appear:
- Vomiting, diarrhea
- Dark or blurred vision due to constricted pupils
- Muscle weakness, twitching, lack of coordination
- Slowed breathing leading to respiratory failure, requiring lifesaving ventilation
- In serious cases, seizures, coma, and death
Pre-operative screening for pseudocholinesterase activity is advised if a person or a close relative has experienced prolonged paralysis and apnea after the use of succinylcholine for anesthesia during an operation.
What does the test result mean?
In monitoring for occupational pesticide exposure
Following exposure to organophosphate compounds, AChE and PChE activity can fall to about 80% of normal before any symptoms occur and drop to 40% of normal before the symptoms become severe. Those who are regularly exposed to these compounds may be monitored for toxic exposure by establishing a baseline activity level and then testing on a regular basis to watch for a significant reduction on activity of acetylcholinesterase or pseudocholinesterase.
In testing for acute pesticide exposure/poisoning
Significantly decreased cholinesterase activity levels usually indicate excessive absorption of organophosphate compounds. Pseudocholinesterase and RBC acetylcholinesterase activity are usually decreased within a few minutes to hours after exposure. Pseudocholinesterase activity may regenerate in a few days to weeks, while acetylcholinesterase activity will remain low for as long as one to three months. Both plasma and RBC activities are immediately affected by pesticide exposure but, upon removal from exposure, AChE and PChE regenerate at different rates since AChE is produced in blood cells, which have a lifespan of 120 days, whereas PChE is produced in the liver, with a half-life of about two weeks.
In testing for succinylcholine sensitivity
About 3% of people have low activity levels of pseudocholinesterase due to an inherited deficiency and will have prolonged effects from the muscle relaxant succinylcholine. Total quantitative pseudocholinesterase levels will be evaluated prior to surgery for patients with a history or family history of prolonged apnea after use of this drug. Low activity levels of pseudocholinesterase levels indicate that these people may be at increased risk of experiencing prolonged effects of the muscle relaxant. A second test, the dibucaine inhibition test, may also be performed to help characterize the degree of a person’s sensitivity to the drug. The lower the result from a dibucaine inhibition test, the greater the risk of drug sensitivity.
Reduced cholinesterase levels can also be caused by chronic liver disease and malnutrition. Total cholinesterase activity can be lowered in a number of other conditions, including pregnancy, renal disease, shock, and some cancers.
Is there anything else I should know?
If someone unexpectedly has prolonged apnea after surgery, testing for succinylcholine sensitivity may be performed; however, the sample should be obtained after 24 to 48 hours have elapsed following the surgery to avoid interference by any drugs given during the surgery that could affect the results.
Drugs called cholinesterase inhibitors may have a moderate benefit in those with early diagnosed Alzheimer disease.
Can I be tested for cholinesterase in my healthcare provider's office?
No, and not every laboratory will offer this testing. It may be necessary to send the sample collected to a reference laboratory.
How long will it take for my results?
That depends on the laboratory performing the testing. Since this is specialized testing and may need to be sent to a reference laboratory, it take a few days for results to be available.