These are contractile proteins of the myofibril. The cardiac isoforms are very specific for cardiac injury and are not present in serum from healthy people. Current guidelines from the American College of Cardiology Committee state that cardiac troponins are the prefered markers for detecting myocardial cell injury.

Troponin I (cTnI) or T (cTnT) are the forms frequently assessed. 

§ Rises 2 - 6 hours after injury
§ Peaks in 12 - 16 hours
§ cTnI stays elevated for 5-10 days, cTnT for 5-14 days

This enzyme is found in heart muscle (CK-MB), skeletal muscle (CK-MM), and brain (CK-BB).  Creatine kinase is increased in over 90% of myocardial infarctions. However, it can be increased in muscle trauma, physical exertion, postoperatively, convulsions, delirium tremens and other conditions.

Time sequence after myocardial infarction

§ begins to rise 4-6 hours
§ peaks 24 hours
§ returns to normal in 3-4 days

Creatine Phosphokinase Isoenzymes

§ MM fraction  - skeletal muscle
§ MB fraction - heart muscle
§ BB fraction - brain

MB fraction

§ Rises and returns to normal sooner than total CK
§ Rises in 3-4 hours
§ Returns to normal in 2 days

CK - MB subforms

This test is becoming more popular. MB2 is released from heart muscle and converted in blood to MB1. A level of MB2 equal or greater than 1.0 U/L and an MB2/MB1 ratio equal or greater than 1.5 indicates myocardial infarction.

Found in striated muscle. Damage to skeletal or cardiac muscle releases myoglobin into circulation.

Time sequence after myocardial infarction

§ Rises fast (2 hours) after myocardial infarction
§ Peaks at 6 - 8 hours
§ Returns to normal in 20 - 36 hours

Have false positives with skeletal muscle injury and renal failure.

This enzyme is no longer used to to diagnose myocardial infarction.