Serum cholinesterase is a test for the enzymes acetylcholinesterase (also known as RBC cholinesterase) and pseudocholinesterase (also known as butyrylcholinesterase or plasma cholinesterase), which act to break down acetylcholine, a chemical critical in the transmission of nerve impulses.

Acetylcholinesterase is found in nerve tissue and red blood cells, and 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 puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. 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:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. A bandage may be applied to the puncture site 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.

Testing for these enzymes is performed to identify exposure to a group of chemicals known as organophosphates (used as pesticides). These chemicals inactivate cholinesterases, and the level of these enzymes serves as an indicator of exposure and risk of toxicity.

A pseudocholinesterase level is also used, although infrequently, to identify liver disease.

This test may also be performed prior to anesthesia with succinylcholine (a paralyzing agent that is eliminated by these enzymes) to exclude a genetic deficiency of these enzymes. This is often done in individuals who have a family history of a prolonged drug effect during anesthesia.

Succinylcholine is one agent used before electroconvulsive therapy (ECT) to prevent a full-blown seizure during treatment. Your health care provider can determine your potential response to succinylcholine with this test.

The normal value of acetylcholinesterase and pseudocholinesterase depends on the technique, and should be determined by the laboratory performing the test. Typically, normal pseudocholinesterase values range between 8 and 18 U/ml.

Note: U/ml = units per milliliter

Decreased pseudocholinesterase levels may be seen in:

• Congenital deficiency
• Poisoning with organophosphates
• Liver damage
• Acute infection
• Chronic malnutrition
• Metastasis
• Myocardial infarction (heart attack)
• Obstructive jaundice

Smaller decreases are seen during:

• Pregnancy
• Use of oral contraceptives

Risks associated with having blood drawn are slight:

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

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.