Cholinesterases hydrolyze concentrated acetylcholine and also cleave other choline esters. Two types of cholinesterase are measured: (1) acetylcholinesterase ("true" cholinesterase) and (2) pseudocholinesterase. Acetylcholinesterase (AcCHS) is found at nerve endings and in erythrocytes; very little is found in serum. Its substrate specificity is limited to acetylcholine, and it is optimally active against very low acetylcholine concentrations. Pseudocholinesterase (PCE) derives from the liver and is normally found in the serum in substantial amounts. It is active against acetylcholine and other choline esters. PCE is unusual in that the diagnostically significant change is depression, not elevation.

An important application of information about PCE is in evaluating individuals for genetic variations of the enzyme before surgery in which succinylcholine, an inhibitor of acetylcholine, is to be used to induce anesthesia. Persons homozygous for the abnormal form of PCE have depressed total serum activity and their enzyme does not inactivate succinylcholine; persons who receive the drug during surgery may experience prolonged respiratory depression. Presence of the abnormal form of PCE is determined by exposing the enzyme to dibucaine. Normal PCE is inhibited by dibucaine, whereas abnormal PCE is found to be "dibucaine resistant."³⁷

• Numerous drugs may falsely decrease cholinesterase levels (e.g., caffeine, theophylline, quinidine, quinine, barbiturates, morphine, codeine, atropine, epinephrine, phenothiazines, folic acid, and vitamin K).

• Suspected exposure to organic phosphate insecticides:
  • Red blood cell AcCHS levels decline with severe exposure; serum PCE decreases occur earlier.
  • When exposure ceases, serum PCE rises before red blood cell AcCHS returns to normal.
  • Red blood cell AcCHS levels are more useful than are serum PCE levels in determining prior exposure.
• Impending use of succinylcholine during anesthesia:
  • Persons homozygous for the abnormal form of PCE have depressed total serum activity, and their enzyme does not inactivate succinylcholine, with the abnormal PCE indicated as "dibucaine resistant."

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

• Because many drugs may alter cholinesterase levels and activity, a medication history should be obtained. It is recommended that those drugs that may alter test results be withheld for 12 to 24 hours before the test, although this practice should be confirmed with the person ordering the study.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Resume any medications withheld before the test.