Serum cholinesterase is a blood test that looks at certain enzymes (acetylcholinesterase and pseudocholinesterase) that help the nervous system work properly.

Acetylcholinesterase (also known as RBC cholinesterase) and pseudocholinesterase (also known as butyrylcholinesterase or plasma cholinesterase) help break down a chemical that nerves need to send signals.

Acetylcholinesterase is found in nerve tissue and red blood cells. Pseudocholinesterase is found primarily in the liver.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

No special preparation is necessary for this test.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

This test is done to determine if a person has been exposed to a group of chemicals known as organophosphates, which are used in pesticides. These chemicals turn off cholinesterases. The level of acetylcholinesterase and pseudocholinesterase in your blood can be used to determine your exposure and risk of toxicity.

This test may also be done, although infrequently, to diagnose liver disease. It may also be ordered before a person receives anesthesia with succinylcholine, which may be given before electroconvulsive therapy (ECT).

Normal values vary from lab to lab. Typically, normal pseudocholinesterase values range between 8 and 18 units per milliliter (U/mL).

Decreased pseudocholinesterase levels may be due to:

• Poisoning from organophosphates (chemicals found in some pesticides)
• Liver damage
• Acute infection
• Chronic malnutrition
• Metastasis
• Heart attack
• Obstructive jaundice

Smaller decreases may be due to:

• Pregnancy
• Use of birth control pills

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)