Creatine kinase-MB subform levels were useful for early exclusion of MI
ACP J Club. 1999 Sept-Oct;131:49. doi:10.7326/ACPJC-1999-131-2-049
Zimmerman J, Fromm R, Meyer D, et al. Diagnostic marker cooperative study for the diagnosis of myocardial infarction. Circulation. 1999 Apr 6;99:1671-7.
What are the diagnostic properties of creatine kinase-MB (CK-MB) subforms, myoglobin, troponin T and I, and total CK-MB activity and mass for myocardial infarction (MI) in patients with chest pain?
Blinded comparison of CK-MB sub-forms, myoglobin, troponin T and I, total CK-MB activity, and total CK-MB mass with late diagnosis by using CK-MB laboratory values (Diagnostic Marker Cooperative Study).
Emergency departments of 4 teaching hospitals in Houston, Texas, USA.
955 patients (mean age 55 y, 60% men) who were ≥ 21 years of age and presented with chest pain of suspected myocardial origin that lasted for > 15 minutes and < 24 hours.
Description of tests and diagnostic standard
Blood was taken on arrival, 1 hour after arrival, every 2 hours for 6 hours after onset of chest pain, and then every 4 hours for 24 hours. Levels of CK-MB subforms, myoglobin, troponin T and I, and total CK-MB activity and mass were measured. The diagnostic standard for MI was a CK-MB mass ≥ 7 ng/mL and a CK-MB index (CK-MB mass/CK) ≥ 2.5% in ≥ 1 samples obtained within 24 hours of hospital arrival.
Main outcome measures
Sensitivity and specificity for the 6 markers.
12.5% of patients had an MI. All markers had high specificities; CK-MB subforms had the highest sensitivity at 6 hours (Table). Measurements of CK-MB activity and troponin I were the most accurate for late diagnosis (> 6 h), particularly with samples taken 10 and 18 hours after onset, respectively (Table).
Creatine kinase-MB subforms effectively ruled out myocardial infarction 6 hours after onset of chest pain.
Sources of funding: In part, Boehringer Mannheim Corporation; Dade International; Helena Laboratories; Spectral Diagnostics, Inc; National Heart, Lung, and Blood Institute.
For correspondence: Dr. R. Roberts, Department of Medicine, Section of Cardiology, 6550 Fannin, MS SM677, Baylor College of Medicine, Houston, TX 77030, USA. FAX 713-790-4348.
Table. Biochemical markers for the diagnosis of myocardial infarction*
|CK-MB subforms||6 h||91.5%||89.0%||8.3||0.1|
|CK-MB activity||10 h||96.2%||97.5%||38.5||0.04|
|Troponin I levels||18 h||95.7%||93.4%||14.5||0.05|
*CK-MB = creatine kinase-MB. LRs defined in Glossary and calculated from data in article.
The use of nontraditional serum markers for early identification of myocardial injury serves 2 purposes. The first is to diagnose myocardial injury in patients with suspected MI and thereby target early aggressive therapy or recognize failure of reperfusion; the second is to rule out MI more quickly in patients who are less likely to have an MI and subsequently avoid prolonged observation in the emergency department or at hospital admission. The study by Zimmerman and colleagues seems to primarily address the latter purpose.
Patients were enrolled consecutively. Of the 119 patients with MI, 41 patients (34%) received thrombolysis or primary percutaneous coronary angioplasty. Presumably, their diagnoses were clear from histories and electrocardiograms. The sensitivity and specificity of the CK-MB subforms for MI in the 78 patients with MI but no reperfusion therapy were not reported separately and may have been different from the 119 patients with MI or the 41 who did not receive reperfusion therapy.
The sensitivity and specificity for each of the markers tested were reported as cumulative values over the whole observation period, although this reporting was not completely clear in the study report.
The authors concluded that CK-MB subforms are useful in excluding MI within 6 hours of onset of chest pain. Because many emergency departments often exclude MI within 6 or 7 hours of onset of pain by using conventional markers (1, 2), use of CK-MB subforms offers no definitive advantage.
James McCord, MD
Steven Borzak, MDHenry Ford Hospital and Medical CenterDetroit, Michigan, USA