Carboxyhemoglobin (COHb)
The share of your hemoglobin that is bound to carbon monoxide instead of oxygen. It shows recent carbon monoxide exposure, including from smoking.
AI-verified against cited sources. Awaiting expert reviewAlso called COHb, HbCO, FCOHbLOINC 20563-3
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What it is
Hemoglobin is the protein in red blood cells that carries oxygen. Carbon monoxide (CO) is a gas you can't see or smell. It comes from burning fuel and from tobacco smoke, and it grabs onto hemoglobin much more tightly than oxygen does. Carboxyhemoglobin is hemoglobin with CO attached, and the result is the percent of your total hemoglobin. Your body also makes a tiny amount of CO on its own.
Why it matters
Hemoglobin that is holding CO can't carry oxygen, so heavy exposure can leave the brain and heart short of oxygen. This test is how doctors confirm CO exposure, for example after a faulty heater, car exhaust or smoke from a fire. It also reflects how much tobacco smoke a person breathes in. A finger pulse oximeter can't tell CO from oxygen, so its reading can look fine even when the blood level is high.
If it’s high
A higher result means more of your hemoglobin is carrying CO. Smoking is a common reason. Breathing fumes from burning fuel is another. The number alone doesn't show how unwell someone is, so symptoms and the situation matter as much as the percent. If you think you may be breathing fumes right now, get to fresh air and call emergency services.
Common causes
- Smoking cigarettes or cigars (the heavier the smoking, the higher the level)
- Breathing fumes from gas or oil heaters, stoves, generators or car exhaust, or smoke from a fire
- Living or working where traffic or air pollution is heavy, which raises the baseline a little
- Faster breakdown of red blood cells, which makes the body produce more of its own CO
- Solvents that contain methylene chloride, such as some paint strippers, which the liver turns into CO
What can help
- If you may be breathing fumes now, move to fresh air and call 911 or your local emergency number
- Ask your clinician where the CO could be coming from and whether the test should be repeated
- Have fuel-burning appliances checked, and use a working CO alarm at home
- If you smoke, ask your clinician about support to quit
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 0–2 %. Source: ARUP Laboratories, Clinical Chemistry and Laboratory Medicine 2021;59(5):947-954, GMS German Medical Science 2021;19:Doc13, Journal of Clinical Medicine 2024;13:2466, University of Iowa Hospitals & Clinics, Pathology Handbook. Labs treat above 17 % as urgent.
What can affect your result
- Smoking: the heavier the smoking, the higher the level
- Where you live and work: busy traffic and polluted air raise it a little
- Time since exposure: about half of the CO clears in 4–5 hours on room air
- Oxygen treatment: breathing oxygen lowers the level much faster, to a half-life of about 1 hour
- Conditions that break down red blood cells faster, which make more CO inside the body
- Sample handling: the sample must stay sealed and free of air
- Smoking raises the level, and the heavier the smoking, the higher it goes (ARUP lists up to 5% at 1–2 packs a day and up to 9% at 3–4 packs a day).
- City traffic and polluted air raise the baseline a little, to about 1–2%.
- Breathing oxygen, including oxygen given by paramedics or in hospital, clears carbon monoxide quickly. A sample drawn after treatment starts can understate the earlier exposure.
- Conditions that break down red blood cells faster, such as hemolytic anemia, make the body produce more of its own carbon monoxide.
- The sample must stay sealed and free of air. ARUP rejects samples in EDTA tubes, samples exposed to room air, and samples with air bubbles or clots, and it does not accept finger-stick or heel-stick blood.
- Standard finger pulse oximeters read carbon monoxide-bound hemoglobin as if it carried oxygen, so they cannot replace this test.
About half of the carbon monoxide clears in 4–5 hours breathing room air, and in about 1 hour breathing 100% oxygen. The level reflects exposure over the past few hours.
When to talk to a clinician
If you might be breathing CO fumes and have a headache, dizziness, nausea, weakness, chest pain or confusion, get to fresh air and call 911 or your local emergency number. A finger pulse oximeter reading in the usual range doesn't rule out CO. For a non-smoker, a level above about 3% is higher than expected, and smokers can reach about 10% without symptoms. If your lab flagged the result as critical, call the clinician who ordered the test today.
Questions you could ask
- Is it worth having my home's fuel-burning appliances checked, and do I have a working CO alarm?
- Could smoking explain this number, or should we look for another source of carbon monoxide?
- Should the test be repeated after I've been away from the possible source?
Questions people ask
Can my finger pulse oximeter tell me if I have carbon monoxide in my blood?
No. A regular pulse oximeter reads CO-bound hemoglobin as if it carried oxygen, so the reading can look fine while the blood level is high. Only a blood test with co-oximetry, or a special multi-wavelength device, measures it.
I smoke. Is a higher result expected?
Yes. Smokers usually run higher than non-smokers. ARUP's report lists up to 5% for 1–2 packs a day and up to 9% for 3–4 packs a day, and a German guideline notes smokers can reach about 10% without symptoms. Your clinician can put your number in context.
Does the percent tell me how ill someone is?
Not reliably. Guidelines say symptoms do not follow the COHb level closely, and the level drops once the person is away from the source or on oxygen. A low level doesn't rule out earlier exposure.
Show the science and sources
Units
- 1 % = 1 %
- 1 % = 0.01 fraction (0–1)
For clinicians and researchers
COHb is reported as a percent of total hemoglobin, measured by multiwavelength spectrophotometry (co-oximetry). CO binds hemoglobin roughly 200 to 250 times more tightly than oxygen does, and it shifts the oxygen dissociation curve to the left, so tissues receive less oxygen. PaO2 stays normal in CO poisoning, and a standard two-wavelength pulse oximeter reads COHb as oxyhemoglobin (a COHb near 70% can still show an SpO2 around 92%), so the level has to be measured in blood. CO diffuses freely, so venous and arterial samples agree and a venous sample is enough. Non-smokers sit around 0.5% from endogenous CO (heme breakdown), 1 to 2% in polluted urban settings, and up to 8 to 10% or more in smokers. Elimination half-life is about 4 to 5 hours on room air and about 1 hour on 100% oxygen. COHb correlates poorly with symptoms and outcome and falls once exposure stops, so a low level does not exclude earlier exposure. Cutoffs vary by source: a German S2k guideline calls 3 to 4% elevated, and a 2024 review treats more than 3 to 5% (more than 10% in smokers) as indicating intoxication. Multi-wavelength CO pulse oximeters exist, but the S2k guideline notes the American College of Emergency Physicians does not currently recommend them for diagnosis. ARUP measures COHb by spectrophotometry on heparinized whole blood kept airtight, whereas Mayo's Carbon Monoxide Exposure test (FCMEB) is a send-out GC/MS assay run by NMS Labs on EDTA blood, with a 7 to 16 day turnaround, so it is a different workflow.
LOINC 20563-3 (primary), 2032-1, 2030-5. Specimen: whole-blood.
Sources
- Carboxyhemoglobin Quant (test 2005757), ARUP Laboratories
- Reference intervals for venous blood gas measurement in adults, Clinical Chemistry and Laboratory Medicine 2021;59(5):947-954 (2021)
- S2k guideline diagnosis and treatment of carbon monoxide poisoning, GMS German Medical Science 2021;19:Doc13 (2021)
- Carbon Monoxide Poisoning: From Occupational Health to Emergency Medicine, Journal of Clinical Medicine 2024;13:2466 (2024)
- Visualization of Critical Limits and Critical Values Facilitates Interpretation, Diagnostics (Basel) 2025;15(5):604 (survey of critical-notification lists from 50 US medical centers, 2024) (2025)
- Critical Decision Thresholds for Urgent Physician Notification of Point-of-Care Testing Results, Diagnostics (Basel) 2026;16(8):1139 (critical-limit lists from 417 US hospitals, 2023-2025) (2026)
- Carbon Monoxide Poisoning, MedlinePlus (U.S. National Library of Medicine)
- Relating oxygen partial pressure, saturation and content: the haemoglobin-oxygen dissociation curve, Breathe 2015;11:194-201 (2015)
- Carbon Monoxide Exposure, Blood (test FCMEB), Mayo Clinic Laboratories (via the Mayo Clinic Laboratories Extended Catalog)
- LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
- Carboxyhemoglobin (test CHB), University of Iowa Hospitals & Clinics, Pathology Handbook
- Critical Value Policy (dated 2025-07-31), Michigan Medicine Laboratories (MLabs) (2025)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.