Toxicology: carbon monoxide monitoring: practice question

A Forensic Medicine question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Forensic Medicine · Toxicology: carbon monoxide monitoring

Sample item

A woman rescued from a closed room with a coal-burning stove has headache and confusion, and her pulse oximeter shows 99%. What is the best way to confirm carbon monoxide poisoning?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Co-oximetry on a blood sample to measure carboxyhaemoglobin

Standard pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so saturation appears falsely normal. Co-oximetry measures carboxyhaemoglobin directly, and 100% oxygen is started at once because it shortens the half-life from about 4 to 6 hours in air to about 1 hour. Arterial oxygen tension is also typically normal.

Option by option

  • A. Rely on the pulse oximeter reading

    Pulse oximetry misreads carboxyhaemoglobin as oxyhaemoglobin.

  • B. Co-oximetry on a blood sample to measure carboxyhaemoglobin (correct)

    Correct. Co-oximetry measures carboxyhaemoglobin.

  • C. Measure the arterial oxygen tension alone

    PaO2 is normal in carbon monoxide poisoning because dissolved oxygen is unaffected.

  • D. Check serum lactate alone

    Lactate may be raised but cannot confirm carbon monoxide exposure.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is B, Co-oximetry on a blood sample to measure carboxyhaemoglobin. Standard pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so saturation appears falsely normal.

Why is "Rely on the pulse oximeter reading" not the answer?

Pulse oximetry misreads carboxyhaemoglobin as oxyhaemoglobin.

Why is "Measure the arterial oxygen tension alone" not the answer?

PaO2 is normal in carbon monoxide poisoning because dissolved oxygen is unaffected.

Why is "Check serum lactate alone" not the answer?

Lactate may be raised but cannot confirm carbon monoxide exposure.

What concept does this question test?

It tests Toxicology: carbon monoxide monitoring in Forensic Medicine. Co-oximetry measures carboxyhaemoglobin directly, and 100% oxygen is started at once because it shortens the half-life from about 4 to 6 hours in air to about 1 hour. Arterial oxygen tension is also typically normal.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Toxicology: carbon monoxide monitoring?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.