Carbon monoxide poisoning — FFICM MCQ

Correct answer: A — Measure carboxyhaemoglobin on co-oximetry

The correct answer is A, measure carboxyhaemoglobin on co-oximetry. This patient has a classic triad for carbon monoxide (CO) poisoning: smoke exposure with soot around the mouth, headache and confusion, a raised anion-gap metabolic acidosis with lactate 6.0 mmol/L, and a co-victim also symptomatic, pointing to a shared environmental toxin rather than an isolated injury. Standard two-wavelength pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so SpO2 remains falsely normal even with severe CO poisoning; a heparinised blood sample analysed by co-oximetry (arterial or venous) is required to measure carboxyhaemoglobin directly and confirm the diagnosis. This result also guides urgency of treatment (high-flow oxygen, hyperbaric oxygen referral) and should be taken as soon as possible, because carboxyhaemoglobin falls with a half-life of around four to six hours on oxygen, so a level taken later may underestimate earlier severity. Why the other options are wrong: D. Rely on pulse oximetry to exclude poisoning: pulse oximetry reads carboxyhaemoglobin as oxyhaemoglobin, giving a falsely reassuring normal SpO2 and cannot exclude CO poisoning. B. Request D-dimer as the primary test: D-dimer screens for venous thromboembolism, not toxic gas exposure, and has no role here. E. Measure methaemoglobin as the primary test: methaemoglobinaemia causes hypoxia with SpO2 plateauing around 85 percent and chocolate brown blood, features absent here; the scenario points to CO, not an oxidant or nitrite exposure. C. Perform spirometry before oxygen therapy: spirometry assesses airway obstruction and would delay urgent oxygen and co-oximetry, inappropriate in suspected CO poisoning. Key point: Normal pulse oximetry never excludes carbon monoxide poisoning; measure carboxyhaemoglobin by co-oximetry whenever smoke inhalation with confusion, headache, acidosis or multiple affected casualties is suspected.

Reference: GOV.UK / UK Health Security Agency (formerly Public Health England), Diagnosing Poisoning: Carbon Monoxide (CO) algorithm, 2015, https://assets.publishing.service.gov.uk/media/5a7568fb40f0b6360e473e60/CO_diagnosis_algorithm_2015.pdf