Carboxyhemoglobin

Test Description

Carbon monoxide (CO) is a colorless, odorless, gaseous substance found in tobacco
smoke, automobile exhaust, fires burning with poor ventilation, improperly functioning
furnaces, and defective gas-burning appliances such as stoves. When the
hemoglobin of the blood is exposed to CO through inhalation, carboxyhemoglobin
is formed. The affinity of hemoglobin for CO is over 200 times greater than for oxygen.
Thus, hemoglobin is prevented from combining with, and transporting, oxygen
to such tissues as the brain. This results in a lack of oxygen being released in the
tissues of the body, a condition known as hypoxia.

Symptoms of CO poisoning vary with the carboxyhemoglobin level. Levels of
20% to 30% cause headache, dizziness, nausea, vomiting, and impaired judgment.
Levels of 30% to 40% result in confusion, muscle weakness, hyperpnea,
hypotension, and tachycardia. When levels reach 50% to 60%, there is a loss of
consciousness and possible seizures, and with values greater than 60%, respiratory

arrest and death may occur.
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THE EVIDENCE FOR PRACTICE

• CO poisoning must be evaluated through testing for CO.
• Arterial blood gases cannot be used to diagnose carbon monoxide (CO) overexposure.
CO does not affect the amount of oxygen dissolved in the serum, only what is attached
to hemoglobin. Thus, PO2 and oxygen saturation will be normal.
• Pulse oximetry monitoring will provide a false normal reading, because the probe will
read the carboxyhemoglobin saturation as oxyhemoglobin.
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Normal Values

                   Nonsmoker:   <3% of total hemoglobin
                   Smoker:       2–10% of total hemoglobin
                   Newborn:     10–12% of total hemoglobin
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Possible Meanings of Abnormal Values
Increased
Carbon monoxide poisoning
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Contributing Factors to Abnormal Values

• Contamination of the blood sample with room air will alter test results.
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Interventions/Implications
Pretest
• Explain to the patient the purpose of the test and the need for a blood sample to be
drawn.
• No fasting is required before the test.
• No smoking is allowed prior to the test.
• Specimen should be drawn as soon as possible after exposure.
Procedure
• A blood sample is collected to fill an EDTA (lavender-top) tube.
• Gloves are worn throughout the procedure.

Posttest

• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued
bleeding.
• The sample is labeled and transported to the laboratory.
• Provide high concentration oxygen to the patient as ordered by the primary care provider.
• Report results to the primary care provider.
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Clinical alert                   

• A person who may have carbon monoxide poisoning should be removed from the
  place of likely exposure and given oxygen before being tested.
• Women and children may have more severe symptoms of CO poisoning at lower
CO levels than men because of the fewer number of red blood cells normally
available to carry oxygen.