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Hemoglobin, hemoglobin M, MetHb, Hgb M.

Common Use:
To assess for cyanosis and hypoxemia associated with polycythemia, pathologies affecting hemoglobin, and potential inhaled drug toxicity.

Whole blood collected in green-top (heparin) tube. Specimen should be transported tightly capped and in an ice slurry.

Normal Findings:
(Method: Spectrophotometry)

Conventional UnitsSI Units (Conventional Units × 155)
0.06–0.24 g/dL9.3–37.2 micromol/L
Percentage of total hemoglobin = 0.41–1.15%.Note: The conversion factor of ×155 is based on the molecular weight of hemoglobin of 64,500 daltons (d), or 64.5 kd.


Methemoglobin is a structural hemoglobin variant formed when the heme portion of the deoxygenated hemoglobin is oxidized to a ferric state rather than to the normal ferrous state, rendering it incapable of combining with and transporting oxygen to tissues. Visible cyanosis can result as levels approach 10% to 15% of total hemoglobin.

This procedure is contraindicated for



  • Assist in the detection of acquired methemoglobinemia caused by the toxic effects of chemicals and drugs.
  • Assist in the detection of congenital methemoglobinemia, indicated by deficiency of red blood cell nicotinamide-adenine dinucleotide (NADH)-methemoglobin reductase or presence of methemoglobin.
  • Evaluate cyanosis in the presence of normal blood gases.

Potential Diagnosis

Increased In:

  • Acquired methemoglobinemia (drugs, tobacco smoking, or ionizing radiation)
  • Carbon monoxide poisoning (carbon monoxide is a form of deoxygenated hemoglobin)
  • Hereditary methemoglobinemia (evidenced by a deficiency of NADH-methemoglobin reductase or related to the presence of a hemoglobinopathy)

Decreased In:

Critical Findings

Cyanosis can occur at levels greater than 10%.

Dizziness, fatigue, headache, and tachycardia can occur at levels greater than 30%.

Signs of central nervous system depression can occur at levels greater than 45%.

Death may occur at levels greater than 70%.

Note and immediately report to the requesting health-care provider (HCP) any critical findings and related symptoms. A listing of these findings varies among facilities. Timely notification of a critical finding for laboratory or diagnostic studies is a role expectation of the professional nurse. The notification processes vary among facilities. Upon receipt of the critical finding, the information should be read back to the caller to verify accuracy.

Possible interventions include airway protection, administration of oxygen, monitoring neurological status every hour, continuous pulse oximetry, hyperbaric oxygen therapy, and exchange transfusion. Administration of activated charcoal or gastric lavage may be effective if performed soon after the toxic material is ingested. Emesis should never be induced in patients with no gag reflex because of the risk of aspiration. Methylene blue may be used to reverse the process of methemoglobin formation, but it should be used cautiously when methemoglobin levels are greater than 30%. Use of methylene blue is contraindicated in the presence of glucose-6-phosphate dehydrogenase deficiency.

Interfering Factors

  • Drugs and other substances that may increase methemoglobin levels include amyl nitrate, aniline derivatives, benzocaine, dapsone, glucosulfone, isoniazid, phenytoin, silver nitrate, and sulfonamides
  • Well water containing nitrate is the most common cause of methemoglobinemia in infants.
  • Breastfeeding infants are capable of converting inorganic nitrate from common topical anesthetic applications containing nitrate to the nitrite ion, causing nitrite toxicity and increased methemoglobin.
  • Prompt and proper specimen processing, storage, and analysis are important to achieve accurate results. Methemoglobin is unstable and should be transported on ice within a few hours of collection, or else the specimen should be rejected.

Nursing Implications Procedure

Related Studies


  • Positively identify the patient using at least two person-specific identifiers before services, treatments, or procedures are performed.
  • Patient Teaching:  Inform the patient this test can assist in identifying the cause of poor oxygenation.
  • Obtain a history of the patient’s health concerns, symptoms, surgical procedures, and results of previously performed laboratory and diagnostic studies. Include a list of known allergens, especially allergies or sensitivities to latex.
  • Note any recent procedures that can interfere with test results.
  • Obtain a list of the patient’s current medications, including over-the-counter medications and dietary supplements (see Effects of Dietary Supplements online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.
  • Prepare an ice slurry in a cup or plastic bag to have on hand for immediate transport of the specimen to the laboratory.


Potential Complications:

  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection online at DavisPlus. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis. The specimen should be placed in an ice slurry immediately after collection. Information on the specimen label should be protected from water in the ice slurry by first placing the specimen in a protective plastic bag.

Post Test

  • Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
  • Instruct the patient to avoid carbon monoxide from firsthand or secondhand smoking, to have home gas furnace checked yearly for leaks, and to use gas appliances such as gas grills in a well-ventilated area.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.
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Bladh, Mickey Lynn., and Anne M. Van Leeuwen. "Methemoglobin." Davis's Lab & Diagnostic Tests, 7th ed., F.A. Davis Company, 2017. Nursing Central, nursing.unboundmedicine.com/nursingcentral/view/Davis-Lab-and-Diagnostic-Tests/425280/all/Methemoglobin.
Bladh ML, Van Leeuwen AM. Methemoglobin. Davis's Lab & Diagnostic Tests. 7th ed. F.A. Davis Company; 2017. https://nursing.unboundmedicine.com/nursingcentral/view/Davis-Lab-and-Diagnostic-Tests/425280/all/Methemoglobin. Accessed April 21, 2019.
Bladh, M. L., & Van Leeuwen, A. M. (2017). Methemoglobin. In Davis's Lab & Diagnostic Tests. Available from https://nursing.unboundmedicine.com/nursingcentral/view/Davis-Lab-and-Diagnostic-Tests/425280/all/Methemoglobin
Bladh ML, Van Leeuwen AM. Methemoglobin [Internet]. In: Davis's Lab & Diagnostic Tests. F.A. Davis Company; 2017. [cited 2019 April 21]. Available from: https://nursing.unboundmedicine.com/nursingcentral/view/Davis-Lab-and-Diagnostic-Tests/425280/all/Methemoglobin.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - Methemoglobin ID - 425280 A1 - Bladh,Mickey Lynn, AU - Van Leeuwen,Anne M, BT - Davis's Laboratory & Diagnostic Tests UR - https://nursing.unboundmedicine.com/nursingcentral/view/Davis-Lab-and-Diagnostic-Tests/425280/all/Methemoglobin PB - F.A. Davis Company ET - 7 DB - Nursing Central DP - Unbound Medicine ER -