USE OF THE COMBINATION OF MYOGLOBIN AND CK-MB MASS FOR THE RAPID DIAGNOSIS OF ACUTE MYOCARDIAL INFARCTION

Kontos MC, Anderson FP, Miller WG et al., Am J Emerg Med, January 1997;15:14.

BACKGROUND: Early identification of patients presenting with myocardial infarction (MI) is necessary for rapid initiation of treatment. Currently, MI has been diagnosed using the combination of the history, electrocardiogram (ECG), and biochemical markers of myocardial necrosis. Unfortunately, all lack sufficient sensitivity and specificity to confidently identify most patients with MI in a timely enough fashion to influence early intervention. Development of newer immunochemical assays for CK-MB mass and myoglobin have allowed for earlier, more rapid diagnosis; however, each has important limitations.

METHODS: The diagnostic sensitivity of CK-MB mass, myoglobin, and the combination of both were analyzed at the time of presentation (0 hours) and again 4 hours later in 101 patients admitted from the emergency department (ED) with possible MI. Twenty patients were subsequently diagnosed as having MI.

RESULTS: The sensitivity of the initial ECG was 60%, compared with the sensitivities of the initial myoglobin and CK-MB mass of 70% and 30%, respectively. By 4 hours the sensitivity of myoglobin had increased to 85% and CK-MB mass to 90%. The combination of the initial myoglobin and CK-MB mass had a sensitivity of 85%. Combining these two markers, using both the initial and 4-hour samples, raised the sensitivity to 100%, with a specificity of 100% and negative predictive value of 100%. When patients with diagnostic ECGs were excluded, the sensitivity of the combination at 0 hours was 80% with a specificity of 84%, while the use of the 0- and 4-hour markers had a sensitivity and specificity of 100% and 100%, respectively.

CONCLUSION: We conclude that the combination of CK-MB mass and myoglobin can rapidly diagnose or exclude MI in as short as 4 hours after ED presentation, and accuracy is not different in patients without diagnostic ECGs. Application of this strategy could potentially lead to more rapid intervention in patients with MI, while also allowing early identification of lower risk patients.

COMMENT: It is impossible to have 100% sensitivity, specificity and negative predictive value. With the low number of patients ruling in for an MI, the 95% CI is very large. This study also only looked at admitted patients, and did not account for patients that were sent home and did have ACS. This may be able to rule out AMI, but not ACS (i.e. unstable angina) - which also requires admission.

EMERGENCY ROOM TRIAGE OF PATIENTS WITH ACUTE CHEST PAIN BY MEANS OF RAPID TESTING FOR CARDIAC TROPONIN T OR TROPONIN I

Hamm CW, et al. N Engl J Med 1997;337(23):1648-1653

BACKGROUND: Evaluation of patients with acute chest pain in emergency rooms is time-consuming and expensive, and it often results in uncertain diagnoses. We prospectively investigated the usefulness of bedside tests for the detection of cardiac troponin T and troponin I in the evaluation of patients with acute chest pain.

METHODS: In 773 consecutive patients who had had acute chest pain for less than 12 hours without ST-segment elevation on their electrocardiograms, troponin T and troponin I status (positive or negative) was determined at least twice by sensitive, qualitative bedside tests based on the use of specific monoclonal antibodies. Testing was performed on arrival and four or more hours later so that one sample was taken at least six hours after the onset of pain. The troponin T results were made available to the treating physicians.

RESULTS: Troponin T tests were positive in 123 patients (16 percent), and troponin I tests were positive in 171 patients (22 percent). Among 47 patients with evolving myocardial infarction, troponin T tests were positive in 44 (94 percent) and troponin I tests were positive in all 47. Among 315 patients with unstable angina, troponin T tests were positive in 70 patients (22 percent), and troponin I tests were positive in 114 patients (36 percent). During 30 days of follow-up, there were 20 deaths and 14 nonfatal myocardial infarctions. Troponin T and troponin I proved to be strong, independent predictors of cardiac events. The event rates in patients with negative tests were only 1.1 percent for troponin T and 0.3 percent for troponin I.

CONCLUSIONS: Bedside tests for cardiac-specific troponins are highly sensitive for the early detection of myocardial-cell injury in acute coronary syndromes. Negative test results are associated with low risk and allow rapid and safe discharge of patients with an episode of acute chest pain from the emergency room.

COMMENT: This study went even further by stating that the 30-day mortality rate is extremely low with negative troponins. However, as before, this study did not really address the diagnosis of unstable angina. When the numbers in this study is used, the sensitivity of two sets of troponin for unstable angina is miserable at only 36% (95% CI 31-42). For 30-day death or delayed MI, the sensitivity is good at 94%, but the 95% CI is 80-99%. Therefore, up to 20% death or delayed MI will occur within the first month - a significant number to be discharged from the ED.