How is the creatine kinase-MB (CK-MB) relative index used as a cardiac marker?

Updated: Jul 30, 2021
  • Author: Kamal (Komo) Gursahani, MD, MBA; Chief Editor: Barry E Brenner, MD, PhD, FACEP  more...
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The relative index calculated by the ratio of CK-MB (mass) to total CK can assist in differentiating false-positive elevations of CK-MB arising from skeletal muscle. A ratio of less than 3 is consistent with a skeletal muscle source, whereas ratios greater than 5 are indicative of a cardiac source. Ratios between 3 and 5 represent a gray zone. No definitive diagnosis can be established without serial determinations to detect a rise.

The CK-MB/CK relative index was introduced to improve the specificity of CK-MB elevation for MI. However, sensitivity for acute MI falls when concurrent cardiac injury and skeletal muscle injury is present. In an ED-based study to evaluate the CK-MB relative index compared with the absolute CK-MB, specificity was increased, but there was a loss of sensitivity. [48]

The CK-MB/CK relative index is useful if patients have only an MI or only skeletal muscle injury, but not if they have both. Therefore, in the combined setting of acute MI and skeletal muscle injury (rhabdomyolysis, heavy exercise, polymyositis), the fall in sensitivity is significant.

Note that the diagnosis of acute MI must not be based on an elevated relative index alone, because the relative index may be elevated in clinical settings when either the total CK or the CK-MB is within normal limits. The relative index is only clinically useful when both the total CK and the CK-MB levels are increased.

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