TnI: Cost management favorable to CK-MB?
TnI: Cost management favorable to CK-MB?
TnI costs $13.06 while CK-MB costs $7.40
The cost-effective management of chest pain patients who present to the emerg-ency department (ED) is an ongoing challenge. Physicians are continually searching for a test to free them from the difficult process of deciding which patients are having an acute myocardial infarction (AMI).
The absence of troponin, when serially tested in a defined evaluation program, greatly lowers the risk of progression to clinical infarction or cardiac death but does not eliminate future risk.1 Typically, healthy people have undetectable levels of TnI in their blood. Cardiac patients may have levels of up to 1.0 ug/L without obvious AMI - levels above 1.5ug/L are indicative. Marginal increases in TnI may be seen in unstable angina.
Unlike creatine kinase-MB (CK-MB), which is found in small amounts in skeletal muscle, cardiac troponin is found only in cardiac muscle. Concurrent skeletal muscle injury in the presence of myocardial damage has sometimes posed a problem with CK-MB since the index could give a false negative value in that situation. Cardiac troponins can help identify false positive CK-MBs and avoid further testing in an attempt to exclude a noncardiac source.
Troponin I (TnI) becomes positive at about the same time as CK-MB does - four to eight hours after an AMI. However, unlike CK-MB, TnI remains elevated for five to nine days after the event, permitting late diagnosis of AMI. CK-MB normalizes two days after the event.
Rapid TnI assays have been shown to be superior to CK-MB tests, so they would seem to be the ideal test to confirm or rule out myocardial damage when the diagnosis is in doubt. Experts disagree, however, even as troponin assays have been improved recently and are becoming routinely available.
Both troponin tests superior to CK-MB
Test results suggest that TnI is superior to TnT, says Michael H. Crawford, MD, chief ofcardiology at the University of New Mexico in Albuquerque. "Sensitivity is generally higher, and there were fewer false-positives, especially due to renal insufficiency." (See related story on renal dysfunction following bypass surgery, p. 51.)
Both troponin tests are superior to CK-MB in the time frame studies (six hours), but older studies have shown that after 12 hours, CK-MB is 99%+ sensitive for AMI and is still the gold standard for diagnosis. Also, CK-MB does not have the false-positive rate in unstable angina that the troponins have. Thus, the real value of the troponins is their ability to rapidly triage patients by six hours after pain onset to determine who need to be admitted and who can go home.
Investigators recently compared the diagnostic accuracy of TnI to the conventional CK-MB assay in ED patients with and without AMI.2 Thirty-five patients diagnosed with AMI were compared to 136 patients diagnosed as not having sustained an infarction using standard criteria of symptoms, EKG abnormalities, and CK-MB level.
The investigators found that TnI offered an advantage over CK-MB only after 24 hours. Within six hours of symptom onset, each test had 40% sensitivity and 98% specificity for the diagnosis of AMI. Among patients presenting at least 24 hours after symptom onset, however, TnI was much more sensitive than CK-MB - 100% vs. 57%, respectively. The authors also found that elevation of either marker was associated with an increased risk of adverse outcomes. The direct laboratory cost of TnI is $13.06, compared to CK-MB's cost of $7.40.
This study is consistent with prior research findings that TnI remains elevated much longer than CK-MB and thus is a better marker of recent infarction, but that neither is useful as a marker of AMI among patients with chest pain of less than six to 12 hours in duration.3,4
References
1. Hlatky MA. Evaluation of chest pain in the emergency department. N Engl J Med 1997; 337:1,687-1,689.
2. Brogan GX, Evaluation of a new assay for cardiac troponin I vs. creatine kinase-MB for the diagnosis of acute myocardial infarction. Acad Emerg Med 1997; 4:6-12.
3. Adams JE, Cardiac troponin-I: A marker with high specificity for cardiac injury. Circulation 1993; 88:101-106.
4. Tucker JF, Early diagnostic efficiency of cardiac troponin I and troponin T for acute myocardial infarction. Acad Emerg Med 1997; 4:13-21.
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