Comparison of carboxyhemoglobin, methemoglobin, and sulfhemoglobin

	Carboxyhemoglobin	Methemoglobin	Sulfhemoglobin
Common causes	Inhaled CO gas from fires, gas-powered generators, kerosene heaters Smoking	Ingested medication (dapsone, antimalarials) Local anesthetic Nitrates or nitrites	Ingested medication (dapsone, phenacetin, sulfonamides) Nitrites
Typical presentation*	Neurologic symptoms from headache to coma Respiratory	Cyanosis Neurologic symptoms from headache to coma Respiratory depression	Cyanosis
Color of the blood	Cherry red	Chocolate brown	Green
Pulse oximetry	Inaccurate	Inaccurate	Inaccurate
Arterial blood gas	PaO₂ normal Carboxyhemoglobin is detected on most machines used in the United States	PaO₂ normal Methemoglobin is detected on most machines used in the United States	PaO₂ normal
Confirmatory testing	Co-oximetry using arterial blood (in stable patients, venous samples are accurate)	Blood gas (arterial or venous)	Requires specialized laboratory testing (spectrophotometry or gas chromatography)
Management	High-flow or hyperbaric oxygen	Methylene blue (MB) or ascorbic acid^¶	Supportive (no antidote) Exchange transfusion in severe cases Lack of response to MB

This table is intended as a quick reference to compare and contrast these three conditions. Some co-oximeters cannot distinguish between methemoglobin and sulfhemoglobin because of their similar absorption peaks, and they may falsely report sulfhemoglobin as methemoglobin. Discussion with laboratory personnel is advised. Refer to UpToDate topics on each of these conditions for details of evaluation and management.

CO: carbon monoxide; PaO₂: arterial partial pressure of oxygen; MB: methylene blue; G6PD: glucose-6-phosphate dehydrogenase.

* Severity of symptoms depends on the magnitude of the intoxication and the rapidity with which it develops. Symptoms may range from mild and nonspecific to life-threatening.

¶ MB is preferred; however, it should not be used in individuals with G6PD deficiency or those taking a serotonergic medication or drug. Refer to UpToDate for details of indications and dosing.

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