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Use of creatine kinase MB isoenzyme for diagnosing myocardial infarction when total creatine kinase activity is high
W G Thompson1, R G Mahr, W S Yohannan
1Department of Medicine, New York University School of Medicine, NY 10016.
Abstract:
The usefulness of measuring creatine kinase MB isoenzyme for diagnosing myocardial infarction when activities of total creatine kinase are very high is unclear. We conducted a retrospective study in an urban hospital that serves a largely indigent population. We concentrated on 146 patients whose creatine kinase activity was greater than 1000 U/L (upper limit of normal: 165 U/L for women and 225 U/L for men), with MB isoenzyme greater than 10 U/L and less than 5% of total creatine kinase. The positive predictive value of MB isoenzyme (isoimmune method) values greater than 10 U/L was between 11.6% and 56.8% when the value for total creatine kinase exceeded 1000 U/L. Using different values (MB greater than 4% of total creatine kinase) as positive for myocardial infarction would have resulted in far fewer false-positives, but 10 cases of myocardial infarction would have been missed. The most appropriate cutoff value for MB isoenzyme in this population (total creatine kinase greater than 1000 U/L) was found to be greater than 2% of total creatine kinase.
Insights
Determining myocardial infarction is challenging when total creatine kinase is very high. A cutoff of >2% for creatine kinase MB isoenzyme offers the best balance of accuracy in this specific patient group.
Area of Science:
- Biochemistry
- Cardiology
- Clinical Diagnostics
Background:
- Elevated total creatine kinase (CK) levels can complicate the diagnosis of myocardial infarction (MI).
- The diagnostic utility of creatine kinase MB isoenzyme (CK-MB) is uncertain in patients with very high total CK activity.
- Previous diagnostic criteria may not be optimal for all patient populations, particularly those with high CK levels.
Purpose of the Study:
- To evaluate the diagnostic performance of CK-MB for myocardial infarction in patients with significantly elevated total CK.
- To determine the most appropriate cutoff value for CK-MB in this specific clinical scenario.
- To assess the impact of different CK-MB thresholds on false-positive and false-negative rates for MI diagnosis.
Main Methods:
- Retrospective study of 146 patients with total CK > 1000 U/L.
- Analysis of CK-MB levels, including absolute values and percentage of total CK.
- Evaluation of positive predictive values for different CK-MB cutoff points.
Main Results:
- Positive predictive value of CK-MB > 10 U/L ranged from 11.6% to 56.8% when total CK exceeded 1000 U/L.
- A cutoff of CK-MB > 4% of total CK reduced false-positives but missed 10 MI cases.
- The optimal cutoff for CK-MB in this population was determined to be > 2% of total CK.
Conclusions:
- Standard CK-MB diagnostic criteria may be unreliable in patients with very high total CK.
- A CK-MB cutoff of > 2% of total CK provides a more appropriate balance between sensitivity and specificity for diagnosing MI in this cohort.
- Further research may be needed to refine diagnostic strategies for myocardial infarction in patients with markedly elevated total creatine kinase.