Unstable angina consists of anginal symptoms but without a diagnostic rise and/or fall of troponin with one level above the 99% reference value for the assay used. It may have ECG findings, or not. Most "unstable angina" is diagnosed in patients who present to the ED with chest pain, or other symptoms suggestive of ischemia, but without a diagnostic ECG and without positive troponins.
After "ruling out for MI," the patient is then "risk stratified" with provocative testing (stress echo or stress sestamibi) or with CT coronary angiogram. However, these are tests of stenosis and do not answer the question "Were the symptoms caused by fissuring and thrombosis of coronary plaque?" In other words, they may answer the question of whether the patient has coronary disease, but not whether that disease caused the symptoms, and thus not whether the patient truly had unstable angina. Stable coronary lesions (stenoses) are much less likely to cause subsequent infarction compared to unstable ones. Evidence of truly unstable angina can be found in the ECG, or even in a rise and fall of troponins that is beneath the 99% reference value.
Patients who are at low risk for coronary disease, have low risk symptoms, have a truly low risk ECG (and this is where ECG expertise really comes into play), and serial undetectable troponins (or detectable but with no rise and fall), have an unknown benefit from such risk stratification testing. (There have been no randomized trials showing benefit.)