Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (31 page)

   UA should be differentiated from
stable angina pectoris
. While similar in character, stable angina pectoris represents short-lived chest discomfort elicited from states of higher cardiac demand that is relieved with rest and is without a crescendo pattern as described above.

   Because of a lack of objective data for diagnosis, unstable angina is the most subjective of the ACS diagnoses. Nevertheless, history/exam, ECG, and cardiac biomarkers on presentation can be utilized to formulate a likelihood of an ACS diagnosis.

   Elderly, diabetic, and female patients are more likely to present with ACS symptoms that do not include chest pain. Rather, symptoms may include dyspnea, diaphoresis, emesis, or hypotension.

   While ACS syndromes are mediated through platelet activation, at present, use of platelet aggregation assays (P2Y12, etc.) and measurement of immature platelet forms are not recommended for the diagnosis of ACS.

   The inflammatory markers/mediators of CRP, serum amyloid A, and IL-6 have been shown to risk stratify UA and NSTEMI patients but cannot be recommended for routine use in clinical diagnosis or guiding therapy for ACS patients at this point in time.

   Who Should Be Suspected of an MI?

   A diagnosis of NSTEMI implies ischemia severe enough to cause myocardial damage as evidenced by a release of cardiac biomarkers of necrosis. While the presence of objective markers of injury makes a diagnosis of MI less prone to error than UA, abnormal values must be interpreted in a clinical context to avoid false-positive interpretation.

   Due to the increasing sensitivity of newer-generation biomarkers of myonecrosis, a universal definition of
myocardial infarction
was adopted in 2007 and specifies that one of the following criteria be met in a clinical setting consistent with myocardial ischemia:

1.   Detection of
rise and/or fall
of cardiac biomarkers (preferably troponin) with at least one value above the 99th percentile of the upper reference limit (URL), together with evidence of myocardial ischemia with at least one of the following:

   Symptoms of ischemia

   ECG changes indicative of new ischemia (new ST-T changes or left bundle branch block, LBBB)

   Development of pathologic Q waves on ECG

   Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality