Carboxyhemoglobin (COHb)

This test measures the fraction of your hemoglobin that is bound to carbon monoxide, called carboxyhemoglobin. Hemoglobin normally carries oxygen, but carbon monoxide attaches very tightly to it, forming COHb and reducing the amount of oxygen your blood can transport.

It is typically performed on a blood sample using a co-oximeter, often as part of a blood gas assessment. The result is reported as a fraction of total hemoglobin, helping your care team understand the extent of recent carbon monoxide exposure from sources such as smoke, engines, heaters, or other fuel-burning appliances.

Carbon monoxide exposure can reduce oxygen delivery to tissues and cause symptoms such as headache, dizziness, nausea, and confusion. At higher exposures, it can be life threatening. The COHb test helps confirm exposure, guides treatment decisions, and monitors how quickly carbon monoxide is clearing with oxygen therapy.

Clinicians order this test when carbon monoxide poisoning is suspected, after smoke inhalation, or to assess baseline levels in people who smoke. It can be especially important for children and during pregnancy, since fetal and pediatric tissues are more sensitive to reduced oxygen delivery.

Your result is interpreted alongside your symptoms, the timing and duration of exposure, smoking status, and whether you received oxygen before the blood draw. Levels typically fall over time, especially if you are breathing fresh air or receiving supplemental oxygen, so timing matters when comparing results.

If your level is higher than expected, your clinician may recommend removing the source of exposure, receiving medical treatment such as oxygen, and repeating the test to confirm that it is improving. If your level is not elevated but you still have concerning symptoms, your care team may look for other causes and consider additional testing. Seek urgent care if you develop severe symptoms or if exposure is ongoing.