Background: The Importance of Cholinesterase Monitoring
Why Monitor Cholinesterase Levels?
- Organophosphate and carbamate pesticides are widely used in developing countries due to cost-effectiveness.
- Despite declining use in developed nations, exposure remains a significant occupational health risk.
- Routine biomonitoring identifies workers at risk and helps implement interventions (e.g., medical treatment, reassignment, improved protective measures).
South African Legal Requirements for Biomonitoring
- Regulated under the “Regulations for Hazardous Chemical Agents 2021” (Occupational Health and Safety Act, Act No. 85 of 1993).
- Employers must monitor and protect workers exposed to cholinesterase inhibitors.
Effective biomonitoring helps prevent acute toxicity and chronic neurological disorders.
Types of Cholinesterase Enzymes
1. Acetylcholinesterase (AChE) - Also Known As:
- True cholinesterase.
- Red blood cell (RBC) cholinesterase.
Location & Function:
- Found in the central nervous system (CNS), neuromuscular junctions, RBC membranes, and the autonomic nervous system.
- Breaks down acetylcholine (ACh), stopping neurotransmission.
- Long half-life (~120 days).
2. Butyrylcholinesterase (BChE) - Also Known As:
- Pseudocholinesterase.
- Plasma cholinesterase.
Location & Function:
- Synthesised in the liver and found in plasma.
- Hydrolyses various choline and non-choline esters (e.g., succinylcholine, mivacurium).
- Shorter half-life (<10 days).
AChE is the preferred biomarker for long-term exposure assessment, while BChE is more relevant in acute poisoning cases.
Pathophysiology of Cholinesterase Inhibition
Normal Cholinesterase Function
- Acetylcholine (ACh) transmits nerve signals at synapses.
- Cholinesterase breaks down ACh into choline and acetate, stopping signal transmission.
How Organophosphates & Carbamates Cause Toxicity
- Mimic ACh and bind irreversibly to cholinesterase enzymes.
- Prevents ACh breakdown, causing excessive nerve stimulation.
- Results in continuous muscle contractions, leading to tremors, paralysis, and respiratory failure.
Acute vs. Chronic Exposure Effects
- Acute poisoning: Causes fasciculations, paralysis, and respiratory failure.
- Chronic low-grade exposure: Linked to neurodevelopmental disorders, lower birth weights, and chronic lung diseases.
Cholinesterase inhibition leads to severe neuromuscular dysfunction, highlighting the need for early detection and intervention.
Cholinesterase Levels in Biomonitoring
Challenges in Establishing Reference Intervals
- Cholinesterase levels vary widely among individuals.
- Baseline measurements before exposure are essential for accurate interpretation.
Recommendations for Biomonitoring in Workers
- Baseline Testing:
- At least two pre-exposure tests, taken 3–14 days apart.
- Results should agree within 15–20%.
- Routine Monitoring During Exposure:
- Periodic testing during peak pesticide application seasons.
- Intervention Thresholds:
- A >30% reduction from baseline suggests toxicity and requires intervention.
Personalised reference levels ensure accurate exposure assessments and timely protective measures.
Best Biomarker for Cholinesterase Inhibition Monitoring
Whole-Blood Cholinesterase is the Most Reliable Indicator
- Measures combined RBC and plasma cholinesterase activity.
- More sensitive for chronic exposure due to RBC cholinesterase’s long half-life (~120 days).
- Provides a comprehensive view of enzyme inhibition.
Why Plasma Cholinesterase is Less Reliable for Monitoring
- Shorter half-life (<10 days) makes it unsuitable for long-term exposure tracking.
- Affected by liver disease, malnutrition, and pregnancy, leading to false readings.
- Still useful for diagnosing acute organophosphate poisoning.
Whole-blood cholinesterase testing is now the preferred method at Ampath for accurate biomonitoring.
Cholinesterase Testing at Ampath
Test Details:
- Sample Type: Whole blood (EDTA tube).
- Testing Method: Colorimetric reaction following manual haemolysis.
- Laboratory: Conducted at Ampath’s National Reference Laboratory (NRL) in the Specialised Chemistry Department.
Ampath offers advanced whole-blood cholinesterase testing for accurate biomonitoring and regulatory compliance.
Key Takeaways for Clinicians & Occupational Health Practitioners
-Cholinesterase monitoring is essential for workers exposed to organophosphate and carbamate pesticides.
-Whole-blood cholinesterase is the superior biomarker for long-term exposure detection.
-Baseline testing is critical to establish accurate reference values.
-A >30% drop from baseline may indicate significant exposure and necessitate intervention.
-Plasma cholinesterase is useful in acute poisoning cases but is not reliable for chronic exposure assessment.
-Ampath provides in-house, high-quality whole-blood cholinesterase testing.
Regular cholinesterase testing ensures workplace safety and compliance with South African health regulations.