A 32-year-old woman is brought to the emergency department 2 hours after ingesting an unknown number of paracetamol tablets in a suicide attempt. She is currently asymptomatic with normal liver function tests. Which investigation is most appropriate to guide immediate management and assess the need for antidote therapy?

Explanation

## Investigation of Choice: Serum Paracetamol Level on Rumack-Matthew Nomogram ### Why This Is the Gold Standard **Key Point:** The Rumack-Matthew nomogram is the only validated tool to predict the risk of hepatotoxicity and determine the need for N-acetylcysteine (NAC) therapy in paracetamol overdose. ### Timing and Interpretation 1. **Optimal timing**: Blood should be drawn **≥4 hours post-ingestion** to allow complete absorption and distribution. 2. **Nomogram zones**: - **High-risk line** (150 µg/mL at 4 hrs): Treat if level is above this line - **Standard-risk line** (100 µg/mL at 4 hrs): Used in patients with risk factors (chronic alcohol use, malnutrition, concurrent enzyme inducers) - **Low-risk line** (50 µg/mL at 4 hrs): Rarely used; for very low-risk patients 3. **Clinical application**: If the level falls above the appropriate line, NAC should be initiated immediately without waiting for liver function tests to become abnormal. ### Why Early Intervention Matters **Clinical Pearl:** NAC is most effective when started within 8 hours of ingestion (>90% efficacy). Waiting for clinical signs of hepatotoxicity (jaundice, coagulopathy, encephalopathy) represents a missed opportunity for prevention — by then, hepatocellular necrosis is already established. **High-Yield:** The nomogram is based on pharmacokinetic studies and predicts which patients will develop fulminant hepatic failure. It is the **only investigation that guides treatment initiation** in the critical early phase. ### Mechanism of Paracetamol Hepatotoxicity Paracetamol is metabolized by the liver via three pathways: - Sulfation (major, dose-dependent) - Glucuronidation (major, dose-dependent) - Cytochrome P450 oxidation (CYP2E1) → **N-acetyl-p-benzoquinone imine (NAPQI)** (toxic metabolite) NAPQI is normally detoxified by glutathione. In overdose, glutathione is depleted, and NAPQI accumulates, causing hepatocellular necrosis. **Mnemonic: SLUDGE-NAC** — Sulfation, Glucuronidation, Oxidation (CYP2E1) → NAPQI → depletion of Glutathione → hepatotoxicity → N-Acetylcysteine (replenishes glutathione and conjugates NAPQI). ### Rumack-Matthew Nomogram Reference | Time Post-Ingestion | High-Risk Level | Standard-Risk Level | Low-Risk Level | | --- | --- | --- | --- | | 4 hours | 150 µg/mL | 100 µg/mL | 50 µg/mL | | 8 hours | 75 µg/mL | 50 µg/mL | 25 µg/mL | | 12 hours | 37.5 µg/mL | 25 µg/mL | 12.5 µg/mL | | 16 hours | 18.75 µg/mL | 12.5 µg/mL | 6.25 µg/mL | | 24 hours | <10 µg/mL | <10 µg/mL | <10 µg/mL | **Tip:** If the patient presents **>24 hours** post-ingestion and is asymptomatic with normal LFTs, the risk of hepatotoxicity is very low, and serum paracetamol level may be undetectable. In this case, measure baseline LFTs and INR; if normal, no NAC is needed.