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Introduction ⬇

Myoglobin (Mb) is the oxygen-binding protein of striated muscle. It resembles Hb but is unable to release oxygen except at extremely low tension. Injury to cardiac or skeletal muscle results in release of Mb. It is not specific to myocardial muscle. Mb is not tightly bound to protein and is rapidly excreted in the urine.

The Mb test is used as an early marker of muscle damage in MI and to detect injury to or necrosis of skeletal muscle. Serum Mb is found earlier than creatine kinase (CK) enzymes in acute MI.

Normal Findings ⬆ ⬇

5–70 ng/mL or 5–70 μg/L

Procedure ⬆ ⬇

  1. Draw a venous blood sample of at least 5 mL (red-topped tube or SST); use serum. Lipemic or grossly hemolyzed specimens are not acceptable. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

  2. Two or three samples taken 1–2 hours apart give optimal results in detecting MI.

Clinical Implications ⬆ ⬇

  1. Increased Mb values are associated with:

    1. Acute MI (elevates 1–3 hours after pain onset, earlier than CK). The amount of Mb correlates with size of infarct

    2. Angina without infarction

    3. Other muscle injury (trauma, exercise, open heart surgery, intramuscular injections, cardiac resuscitation)

    4. Polymyositis and progressive muscular dystrophy

    5. Myositis

    6. Rhabdomyolysis

    7. Inflammatory myopathy (e.g., SLE)

    8. Toxin exposure: narcotic agents, Malayan sea snake toxin

    9. Malignant hyperthermia

    10. Acute kidney injury

    11. Electrical shock

    12. Tonic–clonic seizures

  2. Decreased Mb values are found in:

    1. Circulating antibodies to Mb (many patients with polymyositis)

    2. RA

    3. Myasthenia gravis

Interventions ⬆ ⬇

Pretest Patient Care

  1. Advise patient of test purpose.

  2. Have patient avoid radioisotopes until after blood is drawn.

  3. Avoid vigorous exercise before the test because it may elevate Mb.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Have the patient resume normal activities.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Monitor for MI, muscle inflammation, and metabolic stress.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Clinical Alert

Mb is an early biologic marker of myocardial necrosis. It appears in the peripheral blood 2–3 hours after pain onset and reaches peak levels at 6–9 hours

Interfering Factors ⬆

  1. see Appendix E for drugs that affect test outcomes.

  2. Cocaine use elevates Mb.

  3. Decreased elimination due to kidney insufficiency causes increase of serum levels.