Test Catalog

Test ID: COHBB    
Carbon Monoxide, Blood

Useful For Suggests clinical disorders or settings where the test may be helpful

Verifying carbon monoxide toxicity in cases of suspected exposure

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Carbon monoxide (CO) is a colorless, odorless, tasteless gas that is a product of incomplete combustion of carbonaceous material. CO poisoning causes hypoxia because CO binds to hemoglobin with an affinity 250 times greater than that of oxygen, thus preventing delivery of oxygen to the tissues, but concentrations greater than 20% are associated with symptoms of toxicity (eg, headache, fatigue, dizziness, confusion, nausea, vomiting, increased pulse and respiratory rate). CO levels greater than 50% are potentially fatal. Common exogenous sources of carbon monoxide include cigarette smoke, gasoline engines, and improperly ventilated home heating units. Small amounts of carbon monoxide are produced endogenously in the metabolic conversion of heme to biliverdin. This endogenous production of carbon monoxide is accelerated in hemolytic anemias.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Normal Concentration

Non-Smokers: 0-2%

Smokers: < or =9%

Toxic concentration: > or =20%

Interpretation Provides information to assist in interpretation of the test results

The toxic effects of carbon monoxide can be seen in levels above 20% carboxyhemoglobin. It must be emphasized that the carboxyhemoglobin concentration, although helpful in diagnosis, does not always correlate with the clinical findings or prognosis. Factors other than carboxyhemoglobin concentration that contribute to toxicity include length of exposure, metabolic activity, and underlying disease, especially cardiac or cerebrovascular disease. Moreover, low carboxyhemoglobin concentrations relative to the severity of poisoning may be observed if the patient was removed from the carbon monoxide-contaminated environment a significant amount of time before blood sampling.


An insidious effect of carbon monoxide poisoning is the delayed development of neuropsychiatric sequelae, which may include personality changes, motor disturbances, and memory impairment. These manifestations do not correlate with the length of exposure or with the maximum blood carboxyhemoglobin concentration.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

No significant cautionary statements

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Langman LJ, Bechtel LK, Meier BM, Holstege C: In Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Sixth edition. Chapter 41: Clinical toxicology. Edited by N Rifai, AR Horvath, CT Wittwer. Elsevier; 2018. pp 832-887

2. Disposition of Toxic Drugs and Chemicals in Man. 10th edition. Edited by RC Baselt. Biomedical Publications, 2014