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Carboxyhemoglobin Blood Test

Last updated May 20, 2019

Approved by: Krish Tangella MD, MBA, FCAP

The Carboxyhemoglobin Blood Test helps determine the levels of COHB in blood.

What are other Names for this Test? (Equivalent Terms)

  • Carbon Monoxide Blood Test
  • COHB Blood TestHBCO Blood Test

What is the Carboxyhemoglobin Blood Test? (Background Information)

  • Carboxyhemoglobin (COHB) is the name given to hemoglobin that is bound to carbon monoxide instead of oxygen, the molecule that it normally binds to. This creates a dangerous condition of carbon monoxide poisoning
  • Oxygen is extremely important in sustaining human life. It is so important that fully one-third of the body’s 75 trillion cells are red blood cells, the vehicles for oxygen transport
  • The so-called “bed” of these vehicles, the storage part, is hemoglobin. Hemoglobin is an oxygen-carrying protein that makes up the bulk of red blood cells. Each red blood cell is 97% hemoglobin, meaning that there is a tremendous amount of hemoglobin present at any one time - around 0.75 kg
  • Carbon monoxide, which is released in vehicle exhaust and tobacco smoke among other sources, interferes with hemoglobin’s ability to transport oxygen. This is because hemoglobin preferentially binds to it instead of to oxygen
  • COHB is formed as a result of hemoglobin binding to carbon monoxide, taking oxygen’s place. This is dangerous because the decrease in levels of hemoglobin bound to oxygen impairs the oxygen-carrying capacity of blood. This leads to oxygen deprivation and tissue damage
  • The Carboxyhemoglobin Blood Test helps determine the levels of COHB in blood. It is used to assess the severity of carbon monoxide poisoning and to also gauge the damage caused by cigarette smoking

What are the Clinical Indications for performing the Carboxyhemoglobin Blood Test?

Following are the clinical indications for performing the Carboxyhemoglobin Blood Test:

  • Monitoring the effects of carbon monoxide poisoning
  • Monitoring the effects of cigarette use
  • Muscle fatigue
  • Nausea
  • Vomiting
  • Headache
  • Altered mental status
  • Difficulty breathing

How is the Specimen Collected for the Carboxyhemoglobin Blood Test?

Following is the specimen collection process for Carboxyhemoglobin Blood Test:

Sample required: Blood

Process of obtaining blood sample in adults:

  • A band is wrapped around the arm, 3-4 inches above the collection site (superficial vein that lies within the elbow pit)
  • The site is cleaned with 70% alcohol in an outward spiral, away from the zone of needle insertion
  • The needle cap is removed and is held in line with the vein, pulling the skin tight
  • With a small and quick thrust, the vein is penetrated using the needle
  • The required amount of blood sample is collected by pulling the plunger of the syringe out slowly
  • The wrap band is removed, gauze is placed on the collection site, and the needle is removed
  • The blood is immediately transferred into the blood container, which has the appropriate preservative/clot activator/anti-coagulant
  • The syringe and the needle are disposed into the appropriate “sharp container” for safe and hygienic disposal

Preparation required: No special preparation is needed prior to the test.

What is the Significance of the Carboxyhemoglobin Blood Test Result?

  • A high value for the Carboxyhemoglobin Blood Test may indicate carbon monoxide poisoning

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • Certain factors may interfere with the Carboxyhemoglobin Blood Test. These include diet, exercise, and the presence of hemolytic disorders

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider of the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

The following DoveMed website links are useful resources for additional information:



Please visit our Laboratory Procedures Center for more physician-approved health information:


References and Information Sources used for the Article:

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Jan. 25, 2016
Last updated: May 20, 2019