Cholinesterase Activity

Cholinesterase activity is measured in serum (butyrylcholinesterase, also called pseudocholinesterase or plasma cholinesterase) and in red blood cells (acetylcholinesterase), by kinetic spectrophotometric methods (Ellman method with butyrylthiocholine or acetylthiocholine) or Michel delta-pH assays.

Serum cholinesterase is produced by the liver and falls in hepatic synthetic failure, malnutrition, and with organophosphate and carbamate exposure. Red cell acetylcholinesterase mirrors the enzyme at the neuromuscular junction and synapse and is the better indicator of the degree of poisoning, but is not available in every laboratory.

Dibucaine number and fluoride number identify atypical genetic variants.

Organophosphate and carbamate pesticide poisoning causes about 110 000 to 168 000 deaths yearly, mostly by deliberate self-poisoning in agricultural areas of Asia, Africa and Latin America, with case fatality of 5 to 20%.

These agents inhibit acetylcholinesterase and cause a cholinergic syndrome (bradycardia, bronchorrhoea, bronchospasm, miosis, salivation, lacrimation, urination, diarrhoea, vomiting, fasciculations, weakness, and coma), followed by intermediate syndrome (proximal weakness and respiratory failure at 24 to 96 hours) and delayed neuropathy.

Cholinesterase is also measured before surgery in patients with prolonged paralysis after suxamethonium or mivacurium (1 in 3000 to 1 in 10 000 are homozygous for atypical enzyme), and in workers exposed to pesticides.

Indications. Suspected organophosphate or carbamate poisoning (cholinergic signs, garlic odour, miosis, agricultural worker, unknown poisoning with bradycardia and secretions). Monitoring of pesticide workers (baseline and periodic testing; a fall of 20 to 30% from baseline triggers removal from exposure). Prolonged apnoea or paralysis after suxamethonium or mivacurium. Family studies of cholinesterase variants. Liver synthetic function assessment (rarely). Nerve agent exposure.

Specimen. Serum or heparinised plasma 1 to 2 mL (avoid fluoride and oxalate anticoagulants, which inhibit the enzyme), or EDTA whole blood for red cell acetylcholinesterase (EDTA is acceptable for this assay). Process quickly and keep at 2 to 8 °C; serum is stable for days refrigerated.

Take the sample before treatment with pralidoxime if possible, and record the time of exposure and pesticide name. In occupational testing, take two baseline samples at least 2 weeks apart before exposure.

Reference values. Serum butyrylcholinesterase (37 °C): about 5300 to 12 900 U/L (males 5900 to 12 200, females 4900 to 11 900), with method specific ranges and lower values in pregnancy and in infants. Red cell acetylcholinesterase: about 25 000 to 35 000 U/L of cells, or 28 to 38 units for each gram of haemoglobin (method dependent). Dibucaine number (percentage inhibition): normal 77 to 83%, heterozygous atypical 50 to 70%, homozygous atypical 16 to 25%.

Interpretation.

• Activity under 50% of the lower reference limit (or 20 to 30% fall from the individual's baseline) with cholinergic signs: organophosphate or carbamate poisoning. Severity broadly follows red cell acetylcholinesterase depression: 20 to 50% mild, 10 to 20% moderate, under 10% severe.
• Serum cholinesterase falls rapidly and recovers over 4 to 6 weeks; red cell acetylcholinesterase falls and recovers slowly over 3 months (the red cell lifespan). Carbamate inhibition is reversible and activity returns within hours to days, which may give normal levels when sampling is late.
• Normal activity does not exclude poisoning (low sensitivity in carbamates, delayed sampling, or high baseline), and treatment is clinical.
• Persistently low serum cholinesterase with normal liver tests and a family history: genetic variants (atypical, fluoride resistant, silent), with prolonged paralysis after suxamethonium.
• Low levels in liver disease (cirrhosis, hepatitis), malnutrition, nephrotic syndrome, acute infection, burns, pregnancy, malignancy, and drugs (oral contraceptives, cytotoxics, neostigmine, physostigmine, bambuterol, glucocorticoids), correlate with disease severity and have prognostic value in liver failure.
• Raised cholinesterase: nephrotic syndrome, obesity, diabetes, hyperlipidaemia, alcohol use and thyrotoxicosis, with no clinical effect.
• Serial values show response and relapse; a rising red cell level after oxime treatment shows reactivation (aging of the enzyme limits benefit after 24 to 48 hours with some agents).

Critical values and findings needing immediate action.

• Cholinergic crisis (bradycardia, bronchorrhoea, hypotension, seizures, respiratory distress): immediate atropine (1 to 3 mg intravenously, doubled every 5 minutes until the chest is clear, heart rate over 80 and systolic pressure over 80), oxygen, airway support and ventilation; do not wait for laboratory results.
• Moderate to severe organophosphate poisoning: pralidoxime or other oxime (the benefit is debated and varies by agent), benzodiazepines for seizures, decontamination (remove clothing, wash skin) with protection of staff, and intensive care.
• Intermediate syndrome (neck flexor, proximal and respiratory weakness at 1 to 4 days): ventilatory support and monitoring of vital capacity.
• Prolonged apnoea after suxamethonium: continued ventilation and sedation until recovery (usually 1 to 4 hours, longer in homozygotes), with cholinesterase level, dibucaine number and family testing.
• Occupational fall of 20% or more from baseline (red cell) or 30% (plasma): remove from exposure and investigate.
• Nerve agent exposure: mass casualty procedures, decontamination and antidote auto injectors.

Interferences and pitfalls.

• Wide normal range between individuals, so a single result in a person without baseline is hard to interpret; individual baseline is more useful.
• Serum activity is reduced by many non-toxic causes (liver disease, pregnancy, malnutrition) that mimic poisoning, and red cell acetylcholinesterase is more specific but depends on the assay and haematocrit.
• Samples taken after oxime therapy, or hours after carbamate exposure, may show reactivation or recovery.
• Fluoride, oxalate, citrate and haemolysis interfere; exposure to heat and delay in transport reduce activity.
• Bedside and point-of-care assays are semi-quantitative and used in field screening.
• Different units and methods between laboratories require method specific ranges.
• Atropine and oxime doses follow clinical response (secretions, heart rate, pupils) and not by enzyme level.
• Pesticide identification (labels, container, gas chromatography) helps treatment, since some products have solvents (hydrocarbons) causing aspiration pneumonitis, and some are especially toxic or slow acting (fenthion, dimethoate).
• Delayed deterioration after initial recovery can occur from fat soluble agents, and observation for at least 24 hours is needed.
• Adverse effects of treatment (atropine toxicity with delirium, hyperthermia and ileus; oxime hypertension) require monitoring.

Special groups.

• Children: accidental ingestion of household pesticides, with seizures and coma as presenting features and less typical muscarinic signs.
• Pregnancy: low cholinesterase of pregnancy, and prolonged paralysis with suxamethonium may occur; poisoning is treated with the same measures used for other adults.
• Agricultural workers and sprayers: chronic low level exposure, baseline testing and protective equipment.
• Patients with liver disease: low enzyme activity from decreased synthesis, with risk of prolonged neuromuscular blockade.
• Self-harm: psychiatric assessment and safe storage, which reduce deaths more than treatment advances.
• Regions: highest burden in rural Asia (Sri Lanka, India, China, Bangladesh), Africa and Latin America, where restriction of highly hazardous pesticides has reduced suicide deaths.

Follow up tests. Red cell acetylcholinesterase and serial butyrylcholinesterase, arterial blood gas and lactate, electrolytes, glucose, amylase and lipase, liver function, ECG with QTc, chest radiograph, creatine kinase, creatinine, pesticide identification, dibucaine and fluoride numbers with genotyping and family screening in prolonged apnoea, neurophysiology for delayed neuropathy, and psychiatric assessment.