hemoglobin M

he·mo·glo·bin M

[MIM*142310-various] a group of abnormal Hb's in which a single amino acid substitution favors the formation of methemoglobin in spite of normal quantities of methemoglobin reductase. Strictly speaking, Hb's M are Hb's with mutations at the proximal or distal histidyl residues. Other Hb's M tend to favor the Fe(III) state. Heterozygotes have congenital methemoglobinemia; the homozygous state of these genes is unknown and is presumably lethal. Specific types include: Hb M_Iwate, α^87His→Tyr (α chain, position 87, histidine replaced by tyrosine); Hb M _{Hyde Park}, β^92His→Tyr; Hb M_Boston, α^58His→Tyr; Hb M_Saskatoon, β^63His→Tyr; Hb M_Milwaukee-1, β^67Val→Glu.

Methemoglobin

Synonym/acronym: 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.

Specimen

Whole blood (1 mL) collected in green-top (heparin) tube. Specimen should be transported tightly capped and in an ice slurry.

Normal findings

(Method: Spectrophotometry)

Conventional Units	SI Units (Conventional Units × 155)
0.06–0.24 g/dL	9.3–37.2 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.

Description

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

N/A

Indications

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

N/A

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%.
It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.
Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.
Possible interventions include airway protection, administration of oxygen, monitoring neurological status every hour, continuous pulse oximetry, hyperbaric oxygen therapy, and ex-change transfusion. Administration of activated charcoal or gastric lavage may be effective if performed soon after the toxic agent 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 that may increase methemoglobin levels include acetanilid, amyl nitrate, aniline derivatives, benzocaine, dapsone, glucosulfone, isoniazid, phenytoin, silver nitrate, sulfonamides, and thiazolsulfone.
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 and Procedure

Pretest

Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
Patient Teaching: Inform the patient this test can assist in identifying the cause of poor oxygenation.
Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
Obtain a history of the patient’s hematopoietic and respiratory systems, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
Note any recent procedures that can interfere with test results.
Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
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.

Intratest

Potential complications: N/A
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. 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 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.

Related Monographs

Related tests include alveolar/arterial gradient, blood gases, carboxyhemoglobin, hemoglobin electrophoresis, and pulse oximetry.
Refer to the Hematopoietic and Respiratory systems tables at the end of the book for related tests by body system.