Creatine Kinase MB (CK-MB)

Creatine kinase is composed of two subunits, CK-M (muscle type) and CK-B (brain type), which are combined into three distinct isoenzymes: CK-MM, CK-MB, and CK-BB. The following table illustrates the isoenzyme composition of different tissues.

The skeletal muscle of marathon runners may contain up to 8% CK-MB.  

CK-MB is a more sensitive marker of myocardial injury than total CK activity, because it has a lower basal level and a much narrower normal range. Early medical literature stated that CK-MB levels become elevated in 4 to 6 hours, peaked at 10 to 24 hours, and returned to normal within 3 to 4 days after an acute myocardial infarction. However, these enzyme kinetics were determined using an insensitive electrophoretic method. In 1991, laboratories began measuring CK-MB mass instead of enzyme activity. CK-MB mass assays were able to measure small, but significant changes during the early hours following onset of chest pain.

 In the year 2000, the American College of Cardiology (ACC) and the American Heart Association (AHA) issued new guidelines for the management of patients with unstable angina and non-ST segment elevation myocardial infarction.  At the same time, a joint committee of the European Society of Cardiology and ACC published a consensus document entitled the “Redefinition of MI.”  These articles recommended that troponin T or troponin I should replace CKMB as the preferred cardiac biomarker.

In 2008, Allan Jaffe, a cardiologist at Mayo Clinic published an article entitled, “Requiem for a Heavyweight: the Demise of Creatine Kinase-MB. In this article, he reviewed the reasons that Mayo removed CKMB from its cardiac panel. Essentially, elevated troponin levels were almost completely specific for cardiac injury and had better sensitivity than CKMB.  Mayo Clinic decided that testing for CKMB added only cost and confusion. 

Following this article, CKMB became an obsolete test for the assessment of myocardial infarction. Today, either high sensitivity troponin I or troponin T is used exclusively. 

References

Braunwald E et al: ACC/AHA ACC/AHA Guidelines for the Management of Patients With Unstable Angina and Non–ST-Segment Elevation Myocardial Infarction: Executive Summary and Recommendations, Circulation 2000;102:1193-1209.

Antman E et al. Myocardial infarction redefined—a consensus document of The Joint European Society of Cardiology/American College of Cardiology committee for the redefinition of myocardial infarction: The Joint European Society of Cardiology/ American College of Cardiology Committee, J Amer College Cardiology, 2000;36(3):959-969. 

Saenger AK and Jaffee AS, Requiem for a Heavyweight: The Demise of Creatine-Kinase MB, Circulation, 2008;118(21):2200-2206.