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.
|
Tissue |
CK-MM (%) |
CK-MB (%) |
CK-BB (%) |
|
Brain |
0 |
0 |
100 |
|
Heart |
80 |
20 |
trace |
|
Skeletal muscle |
99 |
1 |
0 |
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. Medical literature commonly states that CK-MB levels become elevated in 4 to 6 hours, peak at 10 to 24 hours, and return 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, most laboratories began measuring CK-MB mass instead of enzyme activity. CK-MB mass assays are able to measure small, but significant changes during the early hours following onset of chest pain.
In 1995, a study of 225 consecutive patients admitted to a Midwestern hospital for evaluation of acute myocardial infarction were reviewed to determine how quickly CK-MB becomes positive.
|
Time of Positive CK-MB |
Number of Cases |
|
O hours |
31 |
|
6 hours |
14 |
|
12 hours |
2 |
|
24 hours |
2 |
Forty nine of the 225 patients had elevated CK-MBs. Interestingly, 45 of the 49 (92%) were detected within 6 hours; only 4 CK-MBs turned positive at 12 or 24 hours. In an additional 13 cases, the 24 hour specimen was canceled because the patient had already been discharged.
The laboratory frequently encounters patients in which the CK-MB is elevated above 6 ng/mL, but the total creatine kinase (CK) is within normal limits (30 - 225 U/L). To determine if these results are clinically significant, the medical records of 85 consecutive patients were reviewed between February and April, 1995. Ten patients were discharged before the clinical outcome could be determined. The remaining 75 patients had the following diagnoses.
|
Diagnosis |
# Patients |
|
Myocardial infarction |
30 |
|
Unstable angina |
15 |
|
Open heart surgery/angioplasty |
12 |
|
Other heart diseases |
10 |
|
Other diseases |
8 |
Other heart diseases included 3 cases of atrial fibrillation, 3 cor pulmonale, 2 cardiomyopathies, 1 aortic valve stenosis, and 1 angina. Other diseases included 3 cases of CNS hemorrhage, 3 pneumonias, 1 bacteremia, and 1 transient ischemic attack with carotid endarterectomy.These results demonstrated that at least 60% of the patients with normal total CK and elevated CK-MB had suffered myocardial damage. Based on these findings, CK-MB should be ordered in symptomatic patients even if total CK is not elevated.
Reference range is 0 - 5 ng/mL. Specimen requirement is one SST tube of blood.
