A 38-year-old man with chronic pain from metastatic cancer is on morphine 60 mg daily. He presents with severe respiratory depression (RR 8/min, SpO₂ 88%) and pinpoint pupils. Which investigation is most appropriate to confirm acute opioid toxicity and guide immediate management?

Explanation

## Investigation of Choice: ABG Analysis ### Why ABG is the Best Investigation **Key Point:** ABG is the most appropriate investigation to assess the severity and type of respiratory depression caused by opioid toxicity and to guide immediate management decisions. ### Clinical Reasoning In acute opioid overdose with respiratory depression, ABG provides: 1. **Immediate assessment of oxygenation and ventilation** - PaO₂ (hypoxemia severity) - PaCO₂ (hypoventilation/CO₂ retention) - pH (respiratory acidosis assessment) 2. **Guides treatment escalation** - Mild hypoxemia → supplemental O₂ - Severe hypoxemia + hypercapnia → intubation + naloxone - Respiratory acidosis severity → determines urgency of ventilatory support ### Why Other Tests Are Not Ideal | Investigation | Why Not First-Line | | --- | --- | | Serum morphine level | Time-consuming (hours); does not guide acute management; levels do not correlate with clinical severity | | Urine drug screening | Qualitative only; does not quantify; does not assess current physiological status | | Serum naloxone level | Naloxone is not routinely measured; not a diagnostic test for opioid toxicity | **Clinical Pearl:** In opioid overdose, clinical diagnosis (respiratory depression + pinpoint pupils + altered consciousness) is sufficient to initiate naloxone. ABG confirms the severity and monitors response to treatment. **High-Yield:** The classic triad of opioid toxicity = respiratory depression + miosis + altered mental status. ABG is the investigation that quantifies the severity of respiratory compromise and guides escalation of care.