CPK-MB Mass is a cardiac marker which assists in the diagnosis of Acute Myocardial Infarction (AMI). This is a very sensitive marker less prone to artifactual elevations as compared to CPK-MB activity. Increase in serum levels occurs between 3-5 hours after the onset of infarction, peaks at 16-20 hours and returns to normal by 48-72 hours. The sensitivity of CPK-MB is dependent upon the time at which a sample is taken, thus it is recommended to draw the sample at 0, 3 & 6 hours after the onset of chest pain. The decline in CPK-MB follows a predictable course & if the decline shows a sudden increase, it is suggestive of re-infarction.