Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (862 page)

   Atypical macro isoenzyme is found primarily in adults who are severely ill with malignancies or liver disease or in children who have myocardial disease. It occurs transiently in about 1% of hospitalized patients and indicates a poor prognosis, except in children.

   Limitations

   
Atypical macro isoenzyme may cause falsely high or low CK-MB results (depending on type of assay), resulting in an incorrect diagnosis of myocardial infarction (MI) or delayed recognition of an actual MI.
The atypical macro isoenzyme is discovered in <2% of all CK isoenzyme electrophoresis studies.

CREATINE KINASE MB (CK-MB)
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   Definition

   CK-MB is the myocardial fraction associated with MI and occurs in certain other states. MB can be used in estimation of infarct size. CK-MB, or CK-MB fraction, is an 84-kDa molecular weight enzyme that represents 40% of the CK present in myocardial tissue. As with total CK, CK-MB typically begins to rise 4–6 hours after the onset of infarction but is not elevated in all patients until about 12 hours. Elevations return to baseline within 36–48 hours, in contrast to elevations in serum troponin, which can persist for as long as 10–14 days. This means that CK-MB, unlike troponins, cannot be used for the late diagnosis of an acute MI but can be used to suggest infarct extension if levels rise again after declining. CK-MB generally comprises a lower fraction of total CK in skeletal muscle than in the heart. As a result, percentage criteria (4%) have been proposed to distinguish skeletal muscle damage from cardiac damage. However, these criteria are not recommended. They improve specificity but do so at the cost of sensitivity in patients who have both skeletal and cardiac injury.

   
Normal range:

   Use

   CK-MB is a widely used early marker for myocardial injury.

   Interpretation

Increased In

   Necrosis or inflammation of cardiac muscle (CK index approximately 2.5%; in all other causes, CK index usually <2.5%):

   AMI.