Cholinesterase Test

• Cholinesterase activity and phenotype studies in a patient can be used to predict the likelihood and degree of post-operative paralysis following the use of the normally short acting muscle relaxants suxamethonium or mivacurium. Dose reduction and greater care with these agents are required if deficiency is identified, so individuals are issued with drug warning cards.
• To monitor exposure to organic phosphorus compounds in the agricultural and chemical industries.

• Pre-operative screening for cholinesterase activity and phenotype is advised if the individual or a close family member has experienced prolonged paralysis after an operation that required ventilation support for several hours after surgery.
• People using specific agents linked to human poisoning including both carbamate (methomyl and aldicarb) and organophosphate (parathion, fenthion, malathion, diazinon, and dursban) insecticides in the farming or chemical industries may be routinely monitored to assess exposure once baseline levels are established. Cholinesterase activity can also be used to assess acute exposure to these compounds which can cause neuromuscular damage.

• Butyrylcholinesterase enzyme activity, biochemical phenotype and genotype may all contribute to interpretation of results.
• Low total enzyme activity in the blood suggests either an atypical enzyme variant which would make that individual susceptible to suxamethonium / mivacurium or acute exposure to organophosphate pesticides.
• The degree of sensitivity to suxamethonium varies according to the patient’s phenotype. Enzyme inhibitor studies are used to determine the biochemical phenotype. This involves incubating the enzyme under standardised conditions with inhibitory agents such as dibucaine and fluoride and assessing the percentage of the enzyme activity that remains (referred to as a ‘number’).
• The enzyme activity and phenotype together can be used to give a risk / degree of suxamethonium sensitivity and need for family studies. Classifications include: ‘usual’ (normal), ‘intermediate’, ‘atypical’ and ‘silent’. The risk of paralysis in individuals with a ‘usual’ phenotype is extremely rare whereas ‘atypical’ or ‘silent’ phenotypes may experience prolonged paralysis (e.g. 2 – 3 hours).
• Warning cards are issued to individuals with likely suxamethonium sensitivity, which should be shown prior to future surgery, as well as recommendations on family screening.
• Following exposure to organophosphate compounds, serum cholinesterase can fall to about 40% before any symptoms occur and up to 70-80% before the symptoms become severe. Direct measurement of red blood cell acetylcholinesterase activity indicates the degree of toxicity, and sequential measurement could be used to assess treatment response. It may also be useful in assessing long-term / occupational exposure. This is not widely available. Butyrylcholinesterase is more easily available but may not correlate with severity of poisoning and cannot be used to guide treatment.
• Genetic (DNA) studies may be useful in giving more detailed information, for example if an ‘atypical’ variant is difficult for the laboratory to identify or if a silent S gene is suspected.

Total cholinesterase enzyme activity can also be lowered in a number of other conditions. Temporary / other causes for decreased enzyme activity should be excluded. These include pregnancy, renal disease, shock, malnutrition, electrolyte abnormalities, neuromuscular disease, medications (e.g. chronic oral contraceptives), burns, anaemia, decompensated heart disease, age and some cancers. These are unlikely to cause severe enzyme deficiency.

As cholinesterase is synthesised by the liver the activity can also be lower in some liver diseases such as acute and chronic hepatitis, advanced cirrhosis and liver metastases. However, normal levels can be found in mild hepatitis and cirrhosis as well as obstructive jaundice.

There may be a risk of a very mild prolonged reaction to suxamethonium in these conditions (minutes as opposed to hours) due to lower activities of the usual enzyme rather than the atypical enzyme variant. If these conditions resolve, enzyme activity will return to normal.

No. It should be tested if the individual or close family member has experienced post-operative paralysis requiring ventilation or if someone has been potentially exposed to organophosphosphate chemicals, for example in the workplace.