A biochemist speaks....
Cardiac enzymes rise in the blood from 4 to 12 hours post-infarct, depending on which ones you measure. If the person concerned died within a few hours of the heart attack, the levels of enzymes would not be raised noticeably, so its a meaningless test. Much better to rely on the gross findings on autopsy. All that elevated cardiac enzyme levels will tell you is that there was an injury to the heart muscle. They don't diagnose what type of injury.
As for how long you can tell. Well it all depends - on when the sample was obtained, how it was stored, and what condition the body was in when the sample was taken. Assuming a correctly stored sample, taken from the body shortly after death, you can determine cardiac enzyme levels months later. The key factor is correct storage of the blood sample (ie the preservative in the tube, and storage at low temperature, and critically, the blood cells must be separated from the serum/plasma).
So come on, enlighten us all. Where did you hide the body?? <img src="wink.gif" border="0">