Carboxyhemoglobin, Capillary (COHb)

This test measures the portion of your hemoglobin that is bound to carbon monoxide, called carboxyhemoglobin. When carbon monoxide attaches to hemoglobin, it reduces the blood’s ability to carry oxygen to your tissues.

A small capillary blood sample, typically from a finger or heel stick, is analyzed by co-oximetry to determine the fraction of hemoglobin that is carrying carbon monoxide. The result reflects how much of your total hemoglobin is affected at the time of sampling.

Carbon monoxide exposure can cause headaches, dizziness, nausea, confusion, and in severe cases, unconsciousness. Because these symptoms overlap with many other conditions, measuring carboxyhemoglobin helps confirm exposure and guide treatment, including supplemental oxygen or evaluation for hyperbaric therapy.

Clinicians order this test after smoke inhalation, suspected exposure from heaters or engines, workplace risks, or when evaluating unexplained symptoms in groups of people from the same environment. It is also used to document baseline levels in people who use tobacco or are exposed to secondhand smoke.

If your result is higher than expected for your situation, it usually indicates recent exposure to carbon monoxide from sources such as indoor combustion, vehicle exhaust, or smoke. Results can change with time and treatment, since levels tend to fall after you leave the exposure or receive oxygen, so timing matters.

Your clinician will interpret the result with your symptoms and other tests to decide on next steps. People who are pregnant, infants, and those with heart or lung disease can be more vulnerable at any given level. If exposure is suspected, your care team may recommend repeat testing, checking others who share the environment, and safety measures to prevent future exposures.