COHb levels can be falsely low if the patient receives oxygen prior to the test. For example, a patient who receives oxygen therapy in the ambulance prior to hospital arrival may have normal or lower-than-expected COHb levels despite carbon monoxide poisoning. Although high COHb levels confirm exposure to carbon monoxide, COHb levels are not always predictive of symptoms or outcome.[6]
An antidote for cyanide poisoning, hydroxocobalamin, is usually administered when co-intoxication with carbon monoxide is suspected. The presence of hydroxocobalamin may interfere with the co-oximetry used to detect COHb, causing falsely low COHb.[16]
Phenobarbital, phenytoin, and progesterone enhance carbon monoxide production, which may affect the level of COHb.[17, 18]
Co-intoxication with cyanide should also be considered in cases of carbon monoxide poisoning due to smoke inhalation and exposure to fires.