Methemoglobin (MetHb)
The share of your hemoglobin in a form that can't carry oxygen well. Labs usually report up to about 1.5%, and higher levels can cause blue-gray skin.
AI-verified against cited sources. Awaiting expert reviewAlso called MetHb, Met-Hb, MetHgbLOINC 2614-6
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What it is
Methemoglobin is a form of hemoglobin that can't carry oxygen the usual way. Hemoglobin is the protein in red blood cells that moves oxygen around your body. A little methemoglobin forms all the time, and an enzyme in your red blood cells keeps turning it back, so only a small share is present at any moment. The result is the percent of your total hemoglobin.
Why it matters
When too much hemoglobin is in this form, less oxygen reaches your tissues. People may have blue-gray skin, headache, tiredness or shortness of breath, and higher levels can be serious. Doctors use this test when oxygen readings or skin color don't match how a person looks, and to check whether a medicine or chemical exposure may be involved.
If it’s high
A higher result means more of your hemoglobin can't carry oxygen well. Small rises, often 3% to 10%, are sometimes found by chance on a blood gas and may cause no symptoms. Larger rises can cause blue-tinged skin, headache and shortness of breath. Causes range from medicines and chemical exposures to inherited conditions. Your clinician can put your result in context.
Common causes
- Certain medicines, such as the antibiotic dapsone, and numbing medicines such as benzocaine (including some sprays and gels)
- Nitrites and nitrates, including some chemicals and food additives
- Industrial chemicals such as nitrobenzene
- Inherited conditions present from birth, such as a missing red blood cell enzyme (cytochrome b5 reductase) or a hemoglobin variant (hemoglobin M)
- Young age: infants are more sensitive because their red blood cells have less of the enzyme that clears methemoglobin
- Inhaled nitric oxide used in hospital, which can cause small rises
What can help
- Tell your clinician about every medicine, spray, gel and supplement you use, including ones you buy without a prescription
- Ask your clinician whether anything you use at home or are exposed to at work could be the trigger
- If blue-tinged skin has been there since birth or runs in your family, ask whether an inherited cause should be checked
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: 0–1.5 %. Source: Michigan Medicine Laboratories (MLabs), send-out to Mayo Clinic Laboratories, Geisinger Medical Laboratories, American Journal of Hematology 2021;96:1666-1678, Clinical Chemistry and Laboratory Medicine 2021;59(5):947-954. Labs treat above 5.1 % as urgent.
Typical signs by methemoglobin level (EHA/EuroBloodNet recommendations, 2021)
- Under 10%: often no symptomsbelow 10 %
- 10% to 30%: blue-tinged skin may show10 to under 30 %
- 30% to 50%: breathlessness, dizziness, fainting30 to under 50 %
- 50% to 70%: serious illness50 to under 70 %
- Over 70%: a medical emergency70 and above %
What can affect your result
- Medicine and chemical exposures, such as dapsone, benzocaine and nitrites
- Age: infants are more sensitive than adults
- Inherited enzyme or hemoglobin variants
- Other conditions that limit oxygen delivery, such as anemia, heart failure or lung disease, can make symptoms worse at the same percent
- Test method: a blood co-oximeter measures it directly, while pulse oximeters cannot
- Some medicines and chemicals raise methemoglobin, including the antibiotic dapsone, numbing medicines such as benzocaine, and nitrites. Tell your clinician about every product you use, including sprays and gels.
- Standard finger pulse oximeters cannot measure methemoglobin. With significant amounts they tend to settle near 85%, whatever the true oxygen level is, so the blood test is needed.
- Sulfhemoglobin, a related pigment, can also cause blue-gray skin. Some labs test for both.
- Handling matters. Mayo's methemoglobinemia evaluation must reach the lab within 72 hours of collection.
When to talk to a clinician
Call 911 or your local emergency number for blue or gray lips, skin or nails together with shortness of breath, dizziness, fainting, chest pain or confusion, especially after a new medicine or chemical exposure. Blue-tinged skin in a baby needs prompt medical attention. In European recommendations, levels above about 10% often show as visible blue discoloration, and levels of 30% or more often cause breathlessness, dizziness or fainting. If your lab flagged the result as critical, call the clinician who ordered the test today.
Questions you could ask
- How does this result fit with my oxygen readings and my hemoglobin?
- Could a medicine, spray, gel or chemical I use be raising my methemoglobin?
- Should we repeat the test to see whether the level is changing?
- If this has been high before or runs in my family, should an inherited cause be checked?
Questions people ask
Why can a pulse oximeter be wrong when methemoglobin is high?
Methemoglobin absorbs light differently from normal hemoglobin. Many pulse oximeters then settle near 85% whatever the true oxygen level is. A blood gas with co-oximetry measures methemoglobin directly.
My result is only a little above the range. Does it matter?
Small rises are sometimes found by chance on a blood gas. In European recommendations these are usually between 3% and 10%, and many people have no symptoms at that level. The cause is often a medicine or chemical exposure. Your clinician can put your result in context.
Show the science and sources
Units
- 1 % = 1 %
- 1 % = 0.01 fraction (0–1)
For clinicians and researchers
Methemoglobin (MetHb) is hemoglobin whose heme iron has been oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, so it cannot bind oxygen and it shifts the oxygen dissociation curve to the left. Red cells normally keep it below about 1% through cytochrome b5 reductase (CYB5R3) and other reducing systems. Acquired forms from oxidant drugs and chemicals are the most common. Inherited forms are CYB5R3 deficiency (types I and II) and the M hemoglobin variants. Results are a percent of total hemoglobin from multiwavelength co-oximetry on a blood gas analyzer, which does not need an arterial sample. Two-wavelength pulse oximeters read about 85% once MetHb is significant and are not a substitute. Visible cyanosis tracks the absolute amount of MetHb (more than about 1.5 g/dL), not just the percent, and anemia, heart failure or lung disease can worsen symptoms at the same percent. Mayo's evaluation (MEV1) adds sulfhemoglobin, cytochrome b5 reductase activity and hemoglobin variant analysis. The 2021 EHA/EuroBloodNet recommendations describe treatment thresholds in acquired cases of about 20% with symptoms and 30% without. Hospital critical-notification lists commonly set the high limit at about 4% to 6%, but individual lists range from 3% to 30%.
LOINC 2614-6 (primary), 2617-9, 2615-3, 15082-1. Specimen: whole-blood.
Sources
- Recommendations for diagnosis and treatment of methemoglobinemia (Joint EHA and EuroBloodNet recommendations), American Journal of Hematology 2021;96:1666-1678 (2021)
- Reference intervals for venous blood gas measurement in adults, Clinical Chemistry and Laboratory Medicine 2021;59(5):947-954 (2021)
- Methemoglobinemia, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
- Methemoglobinemia Evaluation, Blood (test MEV1), Mayo Clinic Laboratories (via the Mayo Clinic Laboratories Extended Catalog)
- 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)
- LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
- Methemoglobin and Sulfhemoglobin, Blood (Mayo Clinic Laboratories test MET), Michigan Medicine Laboratories (MLabs), send-out to Mayo Clinic Laboratories
- Methemoglobin, Whole Blood, Geisinger Medical Laboratories
- Methemoglobin, ART (test AMHB), University of Washington Laboratory Medicine
- 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.