You suspect carbon monoxide poisoning, yet his SaO2 is 98%. How is this possible?

• A. The pulse oximeter is inaccurate.
• B. His hemoglobin is saturated with CO.
• C. His hematocrit is below 25%.
• D. He is bleeding out somewhere.

Answer: (B)

Pulse oximetry can’t tell the difference between hemoglobin bound to oxygen and hemoglobin bound to carbon monoxide. In carbon monoxide poisoning, a large portion of hemoglobin is occupied by CO as carboxyhemoglobin. Carboxyhemoglobin absorbs light in a way that makes the sensor read as if hemoglobin is fully saturated with oxygen, so the SaO2 can be normal or near normal even though oxygen delivery to tissues is seriously impaired. That’s why a normal-looking reading doesn’t rule out CO poisoning—you need CO-oximetry to directly measure carboxyhemoglobin levels.

Other possibilities are less fitting because they don’t explain why the saturation appears normal. An inaccurate device could occur, but the underlying mechanism is the misreading caused by COHb. A very low hematocrit or active bleeding affects blood volume or oxygen-carrying capacity, not the ratio the pulse oximeter reports, so they don’t account for a normal SaO2 reading in this scenario.