Also found in: Dictionary, Acronyms, Encyclopedia.
Related to COHb: carboxyhemoglobinemia, HbCO


Haemoglobin (Hb) with irreversibly bound carbon monoxide (COHb). COHb levels reflect CO exposure, which increases in fires and with exposure to car exhaust; Some COHb occurs naturally as a product of Hb catabolism and due to increased turnover of Hb in newborns, coupled with reduced efficiency of the infant’s respiratory system, leading to increased COHb.

Carboxyhaemoglobin levels
• < 2%: Non-smokers;
• 2%–12%: Smokers;
• > 20%: Toxic;
• > 50%: Lethal.

Carboxyhemoglobin (COHb)

Hemoglobin that is bound to carbon monoxide instead of oxygen.


Synonym/acronym: Carbon monoxide, CO, COHb, COH.

Common use

To identify the amount of carbon monoxide in the blood related to poisoning, toxicity from smoke inhalation, or exhaust from cars.


Whole blood (1 mL) collected in a green-top (heparin) or lavender-top (EDTA) tube, depending on laboratory requirement. Specimen should be transported tightly capped (anaerobic) and in an ice slurry if blood gases are to be performed simultaneously. Carboxyhemoglobin is stable at room temperature.

Normal findings

(Method: Spectrophotometry, co-oximetry)
% Saturation of Hemoglobin
NonsmokersUp to 2%
SmokersUp to 10%


Exogenous carbon monoxide (CO) is a colorless, odorless, tasteless by-product of incomplete combustion derived from the exhaust of automobiles, coal and gas burning, and tobacco smoke. Endogenous CO is produced as a result of red blood cell catabolism. CO levels are elevated in newborns as a result of the combined effects of high hemoglobin turnover and the inefficiency of the infant’s respiratory system. CO binds tightly to hemoglobin with an affinity 250 times greater than oxygen, competitively and dramatically reducing the oxygen-carrying capacity of hemoglobin. The increased percentage of bound CO reflects the extent to which normal transport of oxygen has been negatively affected. Overexposure causes hypoxia, which results in headache, nausea, vomiting, vertigo, collapse, or convulsions. Toxic exposure causes anoxia, increased levels of lactic acid, and irreversible tissue damage, which can result in coma or death. Acute exposure may be evidenced by a cherry red color to the lips, skin, and nail beds; this observation may not be apparent in cases of chronic exposure. A direct correlation has been implicated between carboxyhemoglobin levels and symptoms of atherosclerotic disease, angina, and myocardial infarction.

This procedure is contraindicated for



  • Assist in the diagnosis of suspected CO poisoning
  • Evaluate the effect of smoking on the patient
  • Evaluate exposure to fires and smoke inhalation

Potential diagnosis

Increased in

  • CO poisoning
  • Hemolytic disease (CO released during red blood cell catabolism)
  • Tobacco smoking

Decreased in


Critical findings

    Percent of Total HemoglobinSymptoms
    10%–30%Disturbance of judgment, headache, dizziness
    30%–40%Dizziness, muscle weakness, vision problems, confusion, increased heart rate, increased breathing rate
    50%–60%Loss of consciousness, coma
    Greater than 60%Death
  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • Women and children may suffer more severe symptoms of carbon monoxide poisoning at lower levels of carbon monoxide than men because women and children usually have lower red blood cell counts.

  • A possible intervention in moderate CO poisoning is the administration of supplemental oxygen given at atmospheric pressure. In severe CO poisoning, hyperbaric oxygen treatments may be used.

Interfering factors

    Specimen should be collected before administration of oxygen therapy.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in evaluating the extent of carbon monoxide poisoning or toxicity.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s respiratory system and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Note any recent procedures that can interfere with test results.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Explain to the patient or family members that the cause of the headache, vomiting, dizziness, convulsions, or coma could be related to CO exposure. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain to the patient that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • If carboxyhemoglobin measurement will be performed simultaneously with arterial blood gases, prepare an ice slurry in a cup or plastic bag and have it on hand for immediate transport of the specimen to the laboratory.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture. The tightly capped sample should be placed in an ice slurry immediately after collection. Information on the specimen label should be protected from water in the ice slurry by first placing the specimen in a protective plastic bag.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Recognize anxiety related to test results, and be supportive of impaired activity related to fear of shortened life expectancy. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Educate the patient regarding access to counseling services. Educate the patient regarding avoiding gas heaters and indoor cooking fires without adequate ventilation and the need to have gas furnaces checked yearly for CO leakage. Inform the patient of smoking cessation programs, as appropriate.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include angiography pulmonary, arterial/alveolar oxygen ratio, blood gases, carbon dioxide, CBC, lung perfusion scan, lung ventilation scan, plethysmography, and PFT.
  • Refer to the Respiratory System table at the end of the book for related tests by body system.
References in periodicals archive ?
The values for COex and COHb found in the study are very close to those described by Santos et al.
Hyperbaric oxygen therapy is planned according to the symptoms or blood COHb levels of the patients.
Some 70% of workers in both groups presented COHb figures over 5%, which surpasses the tolerable limits established by the American Conference of Governmental Industrial Hygienists in 2002: 3.
One month later she returned to our clinic and reported that her symptoms resolved; serum COHb was negative upon repeat lab tests.
When the median COHb levels in the patients who had increased cardiac enzymes (n=34) were compared with the group who did not have increased cardiac enzymes (n=291), no statistically significant relation was found (p=0.
A significant improvement after 14 days of e-cigarette smoking was observed for all parameters: arterial COHb (mean reduction 1.
Additional work needed includes monitoring COHb in a standardized manner to assure accuracy; better prediction of severity of exposure; predictive prognosis; developing networks of reporting that include first responders, local county health department, and local emergency management; and the development of outreach materials and techniques for all race/ethnicity populations.
The greater affinity of hemoglobin for CO accounts for the fact that only a small amount of inspired CO causes a large COHb level in the blood.
1] Percent NA NA NA predicted Age first smoked Years of age NA NA NA (range) coHb (range) NA 3.
Significantly higher levels of COHb (haemoglobin with carbon monoxide) were found in waterpipe smokers in comparison with cigarettes smokers.
2]Hb, COHb, HHb, MetHb) to allow consistent, accurate results throughout the hospital, at the point of care, and in the lab.
Factors contributing to this higher risk include (a) CO can cross the placenta, (b) fetal hemoglobin has greater affinity for CO than maternal hemoglobin, (c) the half life COHb in fetal blood is three times longer than that of maternal blood, and (d) the fetus has a high rate of oxygen consumption and lower oxygen tension in the blood than adults (Fierro, O'Rourke, & Burgess, 2001).