Lab Test: Carboxyhemoglobin Level
- 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.
- 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%
- 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.
- 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
- 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
- Specimen relatively stable for at least 2 weeks in filled well-capped (anaerobic) tube
- 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.
Indications & Uses
Storage and Handling
What To Tell Patient Before & After
MESH Terms & Keywords