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Myoglobin is an oxygen carrying heme protein present in high concentrations in the cytoplasm of cardiac and skeletal muscle. Myoglobin has a molecular weight of 18,000 and constitutes 2% of total muscle protein. Because of its low molecular weight and high cellular concentration, elevated blood levels can theoretically be detected prior to other cardiac markers of ischemic injury. Increases may be seen within 2 hours after the onset of chest pain. Peak levels up to10 times the upper limit of normal are reached in 6 to 9 hours. Serum levels return to normal within 24 to 36 hours after infarction. False negative results may occur if myoglobin levels are measured <2 hours before or >24 hours after infarction. A positive myoglobin result within 2 to 12 hours after onset of chest pain needs confirmation, but a negative test is valuable in ruling out acute myocardial infarction. Because myoglobin is present in both skeletal and cardiac muscle, elevated serum levels are not specific for acute myocardial infarction. Elevated serum levels occur after exercise and intramuscular injections. Patients with skeletal muscle damage, renal failure, or shock have elevated levels. Serum creatinine levels need to be measured simultaneously to rule out false positives secondary to renal failure. Our experience with patients admitted to the hospital from the emergency department at indicated that adding serum myoglobin to the Acute Cardiac Injury Panel would increase the cost of patient care, but would not improve upon measurement of CKMB for diagnosis of myocardial infarction. In 25 of 34 patients (75%) both CKMB and myoglobin were elevated in the first sample. In 2 patients, myoglobin was elevated before CKMB, but in five patients an elevated myoglobin was never confirmed by elevated CKMB. Finally, in one patient an early normal myoglobin was confirmed by failure of CKMB to rise, but in another patient an early normal myoglobin was associated with a rising CKMB.
Many of the early claims for increased myoglobin sensitivity were based on a comparison with CKMB measured by electrophoresis. The latest generation of CKMB mass assays appears to be as sensitive as myoglobin in detecting early onset acute myocardial infarction. Reference value is 0 - 0.09 ug/mL. Specimen requirement is one SST tube of blood. |
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