A 28-year-old agricultural worker is found unconscious in a field 4 hours after suspected organophosphate exposure. Clinical examination shows severe miosis, bronchospasm, and bradycardia. Which investigation should be performed immediately to guide acute management and confirm diagnosis?

Explanation

## Acute Management Investigation in Organophosphate Poisoning ### Rationale for Cholinesterase Assay as First Investigation **Key Point:** Serum AND RBC cholinesterase activity levels are the investigations of choice in acute organophosphate poisoning because they: 1. Provide rapid confirmation of diagnosis 2. Guide intensity of antidote therapy (atropine and pralidoxime dosing) 3. Help differentiate organophosphate from carbamate poisoning 4. Establish baseline for monitoring recovery ### Why This Investigation is Chosen in Acute Settings **High-Yield:** In acute poisoning with clinical signs of cholinergic crisis (miosis, bronchospasm, bradycardia, fasciculations), cholinesterase assay: - Confirms the diagnosis within 1–2 hours (faster than metabolite detection) - Quantifies severity: degree of enzyme inhibition correlates with clinical severity - Guides treatment escalation: severe depression (<20% of baseline) mandates aggressive atropinization and pralidoxime - Allows serial monitoring: repeat assays every 4–6 hours track response to therapy ### Comparison of Investigations in Acute Organophosphate Poisoning | Investigation | Timing | Utility in Acute Setting | Guides Treatment? | |---|---|---|---| | **Serum + RBC Cholinesterase** | **1–2 hours** | **Confirms diagnosis, quantifies severity** | **YES — direct correlation** | | Urine Metabolites | 12–24 hours | Retrospective confirmation only | No — delayed | | LFTs & Electrolytes | 1–2 hours | Non-specific, detects complications | No — indirect | | EEG | Variable | Detects seizures if present | No — not diagnostic | ### Clinical Application **Mnemonic:** **CHOP = Cholinesterase for acute Organophosphate Poisoning** **Clinical Pearl:** A single baseline cholinesterase level is essential because: - Individual baseline varies (genetic, age, liver disease, pregnancy) - Diagnosis is made by DEPRESSION from baseline, not absolute value - Serial measurements (0, 4, 8, 24 hours) show recovery trajectory and guide weaning of antidotes ### Interpretation in Acute Context - **RBC-AChE <20% of baseline:** Severe poisoning → Continuous atropine infusion + pralidoxime 1 g IV 6-hourly - **RBC-AChE 20–50% of baseline:** Moderate poisoning → Bolus atropine + pralidoxime - **RBC-AChE >50% of baseline:** Mild poisoning → Supportive care, consider antidotes **Warning:** Do NOT wait for cholinesterase results to start treatment if clinical diagnosis is clear (miosis + bronchospasm + bradycardia = organophosphate until proven otherwise). However, the assay MUST be done to confirm diagnosis and guide therapy intensity.