A 68-year-old man with a history of organophosphate pesticide poisoning presents to the emergency department with severe salivation, bronchospasm, bradycardia, and muscle fasciculations. Which investigation would be most useful to confirm acute cholinergic toxicity and guide the intensity of atropine therapy?

## Diagnosis of Organophosphate Poisoning: Cholinesterase Assay **Key Point:** Plasma and red blood cell (RBC) cholinesterase (pseudocholinesterase and acetylcholinesterase, respectively) levels are the gold standard investigations for confirming acute organophosphate poisoning and assessing severity of enzyme inhibition. ### Why Cholinesterase Levels are the Investigation of Choice Organophosphates irreversibly inhibit acetylcholinesterase (AChE) and butyrylcholinesterase (pseudocholinesterase), leading to accumulation of acetylcholine at synapses and neuromuscular junctions. Measurement of residual enzyme activity: - **Confirms the diagnosis** of organophosphate/carbamate poisoning - **Quantifies severity** of enzyme inhibition - **Guides intensity of treatment** (atropine and oximes) - **Monitors recovery** during therapy ### Two Types of Cholinesterase Assayed | Enzyme | Location | Clinical Use | Interpretation | |---|---|---|---| | **Plasma cholinesterase (Pseudocholinesterase)** | Plasma | Screening, initial assessment | Decreases rapidly; recovers in days to weeks | | **RBC acetylcholinesterase** | Red blood cell membrane | Confirmatory, prognosis | Decreases more slowly; reflects tissue damage; recovers over weeks to months | **High-Yield:** Severity correlation: - **Mild poisoning:** RBC AChE 50–80% of baseline - **Moderate poisoning:** RBC AChE 20–50% of baseline - **Severe poisoning:** RBC AChE <20% of baseline ### Clinical Significance of Cholinesterase Levels **Normal baseline:** RBC AChE ~8–10 units/mL; Plasma cholinesterase ~8–10 units/mL **Diagnostic thresholds:** - **>50% inhibition** = clinical poisoning likely - **>80% inhibition** = severe poisoning; aggressive treatment required - **Plasma cholinesterase <50% baseline** = significant organophosphate exposure **Clinical Pearl:** The **degree of enzyme inhibition correlates with clinical severity** and guides: 1. **Dosing of atropine** — severe inhibition requires higher, more frequent doses 2. **Use of oximes** (pralidoxime) — more effective if given early, before "aging" of enzyme-inhibitor complex 3. **Duration of ICU monitoring** — severe inhibition requires prolonged observation ### Diagnostic and Monitoring Algorithm ```mermaid flowchart TD A[Suspected organophosphate poisoning]:::outcome --> B[Measure plasma & RBC cholinesterase]:::action B --> C{Cholinesterase level?}:::decision C -->|>80% inhibition| D[Severe poisoning]:::urgent C -->|50-80% inhibition| E[Moderate poisoning]:::outcome C -->|20-50% inhibition| F[Mild poisoning]:::outcome D --> G[High-dose atropine + early oximes]:::action E --> H[Moderate-dose atropine + oximes]:::action F --> I[Low-dose atropine + supportive care]:::action G --> J[Repeat cholinesterase at 24-48 hrs]:::action H --> J I --> J J --> K{Recovery trend?}:::decision K -->|Improving| L[Continue therapy, discharge when safe]:::action K -->|Worsening| M[Escalate treatment]:::action ``` **Mnemonic: CHOP-ATE** — **CH**olinesterase **OP**-poisoning **A**ssay guides **T**reatment **E**scalation. ### Why Other Investigations Are Not First-Line **Warning:** Do NOT rely on: - **Serum acetylcholine levels** — acetylcholine cannot be reliably measured in blood (rapidly hydrolyzed); not clinically useful - **Urine metabolites** — non-specific, does not quantify enzyme inhibition, does not guide acute therapy - **EMG** — may show motor unit action potential changes but does not confirm poisoning or guide treatment intensity [cite:KD Tripathi 8e Ch 9; Harrison 21e Ch 477]