EBM Consult

Lab Test: Carboxyhemoglobin Level

    Lab Test
    • Carboxyhemoglobin
    • Measurement of carboxyhemoglobin (COHb) levels in whole blood for the diagnosis and management of carbon monoxide poisoning
    • This test measures the amount of serum COHb, which is formed by the combination of carbon monoxide (CO) and hemoglobin (Hgb).
      • Carbon monoxide unfortunately combines with Hgb 200 times more readily than O2 can combine with Hgb, which results in hypoxemia. Carbon monoxide is colorless, odorless, and tasteless.
    Reference Range
    • Adults:  <2.3% (0.023); <3% saturation of total hemoglobin
      • Smokers:  2.1% - 4.2% (0.021-0.042); other sources suggest: 2% - 5%
      • Heavy smokers (>2 packs/day):  8% - 9% (0.08 - 0.09); other sources suggest: 5% - 10%
    • Hemolytic anemia:  Up to 4%
    • Newborns:  ≥ 10 - 12%
    • Critical Values:  >20%
    Indications & Uses
    • Suspected carbon monoxide poisoning - Carboxyhemoglobin (COHb) levels higher than 5% in a nonsmoker and 10% in a smoker confirm the diagnosis, but correlate poorly with severity of carbon monoxide poisoning. 
    • Potential associations between COHb levels and clinical findings include:
      • COHb 10% - asymptomatic or headache
      • COHb 20% - atypical dyspnea, throbbing headache, dizziness, nausea
      • COHb 30% - severe headache, impaired thinking, disturbed vision
      • COHb 40% - syncope, confusio
      • COHb 50% - lethargy, seizures, coma
      • COHb 60% or higher - cardiopulmonary failure, seizures, coma, death
      • COHb level and clinical manifestation associations are unreliable in many patients because of prehospital delay or oxygen therapy before blood sampling, and/or concomitant cyanide poisoning.
      • Fetal blood COHb levels are about 30% higher than maternal blood levels, because fetal hemoglobin has a higher affinity for CO than does adult hemoglobin.
    Clinical Application
    • Carbon monoxide unfortunately combines with Hgb 200 times more readily than O2 can combine with Hgb, which results in hypoxemia. 
    • CO poisoning is documented by Hgb analysis for COHb.  A specimen should be drawn as soon as possible after exposure, because CO is rapidly cleared from the Hgb by breathing normal air.
    • COHb levels are best determined by a co-oximeter (dedicated spectrophotometer) that simultaneously measures total hemoglobin and fractional (%) values for COHb, oxyhemoglobin, deoxyhemoglobin, and methemoglobin.
    • COHb levels from venous blood samples appear to correlate with those in arterial blood samples.
    • Increased levels indicate:
      • CO poisoning
    Related Tests
    • Oximetry
    • Oxygen saturation - measures the amount of Hgb saturated by O2.
    Drug-Lab Interactions
    • None
    Test Tube Needed
    • Lavender (EDTA) or green (heparin) top tube
    • Obtain the patient history for possible source of CO inhalation
    • Assess the patient for signs and symptoms of mild CO toxicity (e.g., headache, weakness, dizziness, malaise, dyspnea) and moderate to severe CO toxicity (e.g., severe headache, bright-red mucous membranes, cherry-red blood)
    • Maintain patient safety precautions if confusion is present. 
    • Collect 5 mL of arterial or venous blood
    • Do not remove stopper
    • If possible, collect blood before or while starting oxygen
    • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding. 
    • Treat the patient as indicated by the physician.  Usually the patient receives high concentrations of O2.  Severe CO toxicity may be treated with hyperbaric oxygen.
    • Encourage respirations to allow the patient to clear CO from the Hgb
    Storage and Handling
    • Specimen relatively stable for at least 2 weeks in filled well-capped (anaerobic) tube
    What To Tell Patient Before & After
    • Explain the procedure to the patient or the family.
    • Obtain the patient history for possible source of CO inhalation.
    • Varon J et al. Carbon monoxide poisoning: a review for clinicians. J Emerg Med 1999;17(1):87-93.
    • Prockop LD et al. Carbon monoxide intoxication: an updated review. J Neurol Sci 2007;262:122-130.
    • Mahoney JJ et al. Measurement of carboxyhemoglobin and total hemoglobin by five specialized spectrophotometers (co-oximeter) in comparison with reference methods. Clin Chem 1993;39(8):1693-700.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • Carboxyhemoglobin level, COHb level