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Was it a heart attack?

Editor—McKenna and Forfar describe how the proposed redefinition of myocardial infarction has made the measurement of cardiac specific troponin central to its diagnosis.1 In a patient presenting with chest pain a troponin concentration above the 99th centile of normal is now sufficient to diagnose myocardial infarction, irrespective of any electrocardiographic changes. Previously, as the authors state, a patient had to show the development of Q waves on electrocardiography or an increase in creatine kinase activity to more than twice the upper reference limit before this judgment was made. The widespread introduction of troponin T and troponin I measurements is an undoubted improvement, but limitations exist with the assays, which most clinicians are unaware of and may give rise to diagnostic difficulties. 
 
One issue concerns the use of the 99th centile of normal as the cut off point for myocardial infarction. Most assays are not sensitive enough to measure values as low as this. For example, the 99th centile for troponin T (Roche Diagnostics, Lewes, UK) is about 0.01 μg/l, but the laboratory assay in routine use is unable to measure reliably below 0.03 μg/l. This means that patients with troponin T concentrations between 0.01 μg/l and 0.03 μg/l, who would be defined by the new criteria as having had a myocardial infarction, are currently being missed and included in the low risk category. McKenna and Forfar mention the use of bedside as opposed to laboratory troponin measurement, but near patient tests are less sensitive again, with the troponin T example being suitable for measurement only down to 0.1 μg/l. 
 
A second issue is that troponin measurement involves immunoassay analytical techniques as opposed to the more robust enzymatic methods used with traditional cardiac markers such as creatine kinase. As Ismail and Barth pointed out, immunoassay methods can—unpredictably—lead to wrong results in some people and in the context of troponins these are likely to be difficult to identify.2 
 
These and other limitations of troponin assays should not detract from the value they provide in most patients tested. They do raise concerns, however, if these tests are being used as the sole means of diagnosing myocardial infarction in a patient presenting with chest pain.