Clinical Pathology: General Principles, Clinical Chemistry

• Myoglobin is a relatively small (17.8 kDa) heme-containing protein. The primary function of myoglobin is to store oxygen in striated muscle for release during times of oxygen deprivation.

• Myoglobin is present in both cardiac and skeletal muscle and, therefore, is not a specific cardiac marker. Because of its low molecular weight, myoglobin is rapidly released by cells into the circulation and is the first marker to be elevated after an acute myocardial infarction.

• After an acute myocardial infarction, the serum levels of myoglobin increase between 1 and 4 hours, peak between 5 and 9 hours, and return to baseline levels between 24 and 36 hours. Serial determinations of myoglobin improve the predictive value of using a single myoglobin measurement to identify cardiac muscle injury.

• The advantages of measuring myoglobin are that it is elevated in serum before other cardiac markers and that myoglobin serum concentration is dependent on the amount of cardiac damage.

• The disadvantage of measuring myoglobin in the setting of a suspected acute myocardial infarction is that myoglobin is not a specific marker for cardiac necrosis; it is also increased in any condition where there is skeletal muscle damage (e.g., after cardiopulmonary resuscitation) and in renal failure.

• A normal serum myoglobin value has a strong negative predictive value for an acute myocardial infarction in a patient admitted to the emergency department 6 hours after the onset of chest pain.

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