Type of test Blood
Normal findings
Serum cholinesterase: 8-18 units/mL or 8-18 units/L (SI units)
RBC cholinesterase: 5-10 units/mL or 5-10 units/L (SI units)
Dibucaine inhibition: 79%-84%
(Values vary with laboratory test methods.)
Test explanation and related physiology
This test is done to identify patients with pseudocholinesterase deficiency before anesthesia or to identify patients who may have been exposed to phosphate poisoning. Cholinesterases hydrolyze acetylcholine and other choline esters and thereby regulate nerve impulse transmission at the nerve synapse and neuromuscular junction. There are two types of cholinesterases: acetylcholinesterase, also known as true cholinesterase, and pseudocholinesterase. True cholinesterase exists primarily in the RBCs and nerve tissue. It is not in the serum. Pseudocholinesterase, on the other hand, exists in the serum. Deficiencies in either of these enzymes can be acquired or congenital.
Because succinylcholine (the most commonly used muscle relaxant during anesthesia induction) is inactivated by pseudo cholinesterase, people with an inherited pseudocholinesterase enzyme deficiency exhibit increased and/or prolonged effects of succinylcholine. Prolonged muscle paralysis and apnea will occur after anesthesia in these patients. This situation can be avoided by measuring serum cholinesterase (pseudocholinesterase) in all patients with a family history of prolonged apnea after surgery.
Because patients with a nonfunctioning variant of pseudo cholinesterase will have normal total quantitative pseudocholinesterase levels yet still have prolonged paralytic effects of succinylcholine, a second test (dibucaine inhibition) usually is also performed. Dibucaine is a known local anesthetic that inhibits the function of normal pseudocholinesterase. The dibucaine inhibition number is the percent of pseudocholinesterase activity that is inhibited when dibucaine is added to the patient’s serum sample. If total pseudocholinesterase is normal and dibucaine numbers are low, the presence of a nonfunctioning pseudocholinesterase variant is suspected, and the patient will be at risk for succinylcholine-induced prolonged paralysis.
A common form of acquired cholinesterase deficiency, either true or pseudocholinesterase, is caused by overexposure to pesticides, organophosphates, or nerve gas. The half-life of the pseudoenzyme in serum is about 8 days, and the true cholinesterase (acetylcholinesterase [AChE]) of RBCs is more than 3 months (determined by erythropoietic activity). Recent exposure up to several weeks is determined by assay of the pseudoenzyme and months after exposure by measurement of the red cell enzyme. Persons with jobs associated with chronic exposure to these chemicals are often monitored by the frequent testing of RBC cholinesterase activity. Other potential causes of reduced cholinesterase activity include chronic liver diseases, malnutrition, and hypoalbuminemia.
Increased cholinesterase activity, when found in the amniotic fluid, represents strong evidence for a neural tube defect (NTD). When an NTD is suspected based upon maternal serum alpha fetoprotein (AFP) screening results or diagnosed via ultrasound, analyses of AFP and AChE in amniotic fluid are useful diagnostic tools.
Interfering factors
• Pregnancy decreases test values.
* Drugs that may cause decreased values include atropine, caffeine, codeine, estrogens, morphine sulfate, neostigmine, oral contraceptives, phenothiazines, quinidine, theophylline, and vitamin K.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: no
• Blood tube commonly used: red
• If the test is done to identify the presurgical patient who may be at risk for cholinesterase deficiency, be sure that the test is completed several days before the planned surgery.
• It may be recommended to withhold medications that could alter test results for 12 to 24 hours before the test.
Abnormal findings
Increased serum levels
- Hyperlipidemia
- Nephrosis
- Diabetes mellitus
Increased RBC levels
- Reticulocytosis
- Sickle cell disease
Decreased serum levels
- Poisoning from organophosphate insecticides
- Hepatocellular disease
- Persons with congenital pseudocholinesterase enzyme deficiency
- Malnutrition
Decreased RBC levels
- Congenital cholinesterase deficiency
- Poisoning from organophosphate insecticides
