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's disease.