The cholinesterase of serum is referred to as pseudocholinesterase to distinguish it from the true cholinesterase of the RBC. Both of these enzymes act on acetylcholine and other cholinesters. Alkylphosphates are potent inhibitors of both serum and RBC cholinesterase.

Patients who are homozygous for the atypical gene that controls serum cholinesterase activity have low levels of cholinesterase that are not inhibited by dibucaine. Persons with normal serum cholinesterase activity show 70%–90% inhibition by dibucaine (an amino amide). The red cell (true cholinesterase) enzyme is specific for the substrate acetylcholine.

These are two separate tests. The primary use of serum cholinesterase measurement (pseudocholinesterase) is to monitor the effect of muscle relaxants (e.g., succinylcholine), which are used in surgery. Patients for whom suxamethonium anesthesia is planned should be tested using the dibucaine inhibition test for the presence of atypical cholinesterase variants that are incapable of hydrolyzing this widely used muscle relaxant.

The RBC cholinesterase test is used when poisoning by pesticides such as parathion or malathion is suspected. Severe insecticide poisoning causes headaches, visual distortions, nausea, vomiting, pulmonary edema, confusion, convulsions, respiratory paralysis, and coma.

Acetylcholinesterase: 7 ± 3.8 (SD) U/g Hb or 2 ± 0.2 mU/mol Hb

Serum cholinesterase: 4.9–11.9 U/mL or 4.9–11.9 kU/L

Dibucaine inhibition: 79%–84% or 0.79–0.84

Fluoride inhibition: 58%–64% or 0.58–0.64

RBC cholinesterase: 30–40 U/g Hb

Values vary with substrate and method. These are two different tests. Values are low at birth and for the first 6 months of life.

Procedures

1. For serum cholinesterase, obtain a 5-mL blood sample; 3 mL of serum is needed. This is stable for 1 week at 39 °F–77 °F or 4 °C–25 °C. Observe standard precautions.

2. For RBC cholinesterase, draw a blood sample using sodium heparin as an anticoagulant; do not use serum. Observe standard precautions. This is stable for 1 week at 39 °F–77 °F or 4 °C–25 °C.

1. Decreased or no serum cholinesterase occurs in the following conditions:

   1. Congenital inherited recessive disease. These patients are not able to hydrolyze drugs such as muscle relaxants used in surgery. These patients may have a prolonged period of apnea and may die if they are given succinylcholine

   2. Poisoning from organic phosphate insecticides

   3. Liver diseases, hepatitis, cirrhosis with jaundice

   4. Conditions that may have decreased blood albumin, such as malnutrition, anemia, infections, skin diseases, and acute MI

   5. Heart failure

2. Decreased RBC cholinesterase levels occur in the following conditions:

   1. Congenital inherited recessive disease

   2. Organic phosphate poisoning

   3. Paroxysmal nocturnal hemoglobinuria

   4. Megaloblastic anemia (returns to normal with therapy)

3. Increased serum cholinesterase is associated with:

   1. Type IV hyperlipidemia

   2. Nephrosis

   3. Obesity

   4. Diabetes

4. Increased RBC cholinesterase is associated with:

   1. Reticulocytosis (increase in immature RBCs or reticulocytes)

   2. Sickle cell anemia

   3. Hemolytic anemias

5. Increased RBC cholinesterase in amniotic fluid, along with elevated α-fetoprotein, is presumptive evidence of open neural tube defect (not normally present in amniotic fluid).

Pretest Patient Care

1. Explain test purpose and procedure.

2. Draw blood for serum cholinesterase 2 days before surgery.

3. Be aware that blood should not be drawn in the recovery room; prior administration of surgical drugs and anesthesia invalidates the test results.

4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.