Monitoring: pulse oximetry in CO poisoning: practice question

A Anaesthesia 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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Anaesthesia · Monitoring: pulse oximetry in CO poisoning

Sample item

A smoke inhalation victim has an SpO2 of 99% but is confused and tachypnoeic. What explains the normal reading?

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

The answer and the reasoning

D. Carboxyhaemoglobin is read as oxyhaemoglobin

Carboxyhaemoglobin absorbs red light like oxyhaemoglobin, so the oximeter overestimates saturation. Co-oximetry on arterial blood is needed to detect it.

Option by option

  • A. Nail polish blocks the reading

    Nail polish usually lowers readings.

  • B. Fetal haemoglobin interferes

    Fetal haemoglobin has little effect.

  • C. Methaemoglobin lowers the reading

    Methaemoglobin pushes readings towards 85%.

  • D. Carboxyhaemoglobin is read as oxyhaemoglobin (correct)

    Correct. COHb mimics oxyhaemoglobin.

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 Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is D, Carboxyhaemoglobin is read as oxyhaemoglobin. Carboxyhaemoglobin absorbs red light like oxyhaemoglobin, so the oximeter overestimates saturation.

Why is "Nail polish blocks the reading" not the answer?

Nail polish usually lowers readings.

Why is "Fetal haemoglobin interferes" not the answer?

Fetal haemoglobin has little effect.

Why is "Methaemoglobin lowers the reading" not the answer?

Methaemoglobin pushes readings towards 85%.

What concept does this question test?

It tests Monitoring: pulse oximetry in CO poisoning in Anaesthesia. Co-oximetry on arterial blood is needed to detect it.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Monitoring: pulse oximetry in CO poisoning?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia 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.