In order to distinguish CO poisoning from other indoor air pollutants, thorough history-taking is critical. Important points to identify include living situation (age of the house, use of flame-based heating elements (e.g. oil, propane, gasoline, natural gas) or gas-fired electricity generators) and others in the household having a similar constellation of symptoms.

Diagnosis is based on a suggestive history and physical findings coupled with confirmatory testing. Patients with significant COHgb levels can have normal readings on a regular pulse oximeter. Confirmatory methods include direct measurement of blood COHgb or measurement of COHgb using a transcutaneous co-oximeter capable of specifically measuring COHgb.^9,28 Normal COHgb levels in non-smokers can range from 1-2%, while in cigarette smokers, they can range from 5-10%.^23,25 Patients should be examined for other conditions, including smoke inhalation, trauma, medical illness, or intoxication. A neurological exam should include an assessment of age-appropriate cognitive function.