When admitting a patient to rule out myocardial infarction, just about every ER and hospital in this nation draws a CPK, CK-MB, and troponin-I level every 6-8 hrs (some draw myoglobin levels too). There is no such thing as a "best" enzyme to use, because in the clinical setting, all three enzymes are used, and all are very useful in making clinical decisions about patients. A patient with chest pain and an elevated CPK only may be heparinized and admitted to a higher level of care then a patient with a normal CPK on presentation because of the higher likelihood that he or she is having an acute MI despite the fact that CPK is very non-specific for MI's. They used to use CK-MB levels to formally diagnose MI's for clinical trial purposes, but they have been using troponin-I's more often as the gold standard these days because although CK-MB is very sensitive and fairly specific, skeletal muscles also have a small amount of CK-MB so you can see a small rise in that level (enough to reach the MI "criteria") when you have something like rhabdomyolysis. That's one reason why they also look at the proportion of CK-MB and CPK to one another. Troponin-I levels are the most specific for diagnosing an MI, and very sensitive by 24 hrs; but because they take a while to rise, we still draw CK-MB's and CPK's on all of our patients too. Anyways, for USMLE purposes, I would definitely know the timeline of when the enzymes are released, and I would also know that troponin-I is the most specific test for diagnosing an MI.
