When Is Creatine Kinase and CKMB considered significant in Acute Myocardial Infarction?

I read two interesting articles that mentioned about the biochemical markers in AMI.


Question 1: When Is Creatine Kinase and CKMB considered significant in Acute Myocardial Infarction?

Creatinine Kinase (CK) and Creatine Kinas-MB (CK-MB) is significant when [in the presence of other supportive criteria like typical chest pain, etc]

1. CK elevated ≥ 2x upper laboratory normal and/or

2. CK-MB elevated ≥ 7% if aetiology of the total creatine kinase was equivocal or

3. If the CK elevation <2x upper laboratory normal but there is accompanying serial ECG changes suggested of AMI

Reference:
Menown IB, Mackenzie G, Adgey AA. Optimizing the initial 12-lead electrocardiographic diagnosis of acute myocardial infarction. Eur Heart J 2000; 21 (4):275-83.

Question 2:
Why is using CK and CK-MB less sensitive compared to the cardiac Troponins in the aid of the diagnosing AMI?

Cardiac specific Troponins may rise following episodes of micro-necrosis (minor myocardial damage) which may be undetected by CK or CK-MB.

Reference:
Menown IB, Mackenzie G, Adgey AA. Optimizing the initial 12-lead electrocardiographic diagnosis of acute myocardial infarction. Eur Heart J 2000; 21 (4):275-83.

Question 3

Why is CK-MB or myoglobin rises earlier compared to the cardiac troponins in myocardial infarction?

Myoglobin and CK-MB are proteins present in cytosol (cytosolic proteins) compared to troponins which are the structural proteins.

Cytosolic proteins are released more rapidly after the onset of symptoms compared to structural proteins.

In GUSTO IIA Troponin substudy, it was found that only 36% had increased cardiac troponin T at baseline, and two thirds of the patients who subsequently exhibited cardiac troponin T did not do so until 16 hours after onset.

For optimal triage and risk stratification in ED, therefore, early markers of myocardial injury, i.e. CK-MB subforms and myoglobin appear preferable to avoid unncessary delays such as may be required for the cardiac troponin markers

On the hand, cardiac troponins should be measured serially for its prognostic benefit

Reference:
Roberts R, Fromm RE. Management of acute coronary syndromes based on risk stratification by biochemical markers: an idea whose time has come. Circulation 1998; 98 (18):1831-3.


Further readings:

1. Menown IB, Mackenzie G, Adgey AA. Optimizing the initial 12-lead electrocardiographic diagnosis of acute myocardial infarction. Eur Heart J 2000; 21 (4):275-83.
Download article: http://eurheartj.oxfordjournals.org/cgi/reprint/21/4/275

2. Roberts R, Fromm RE. Management of acute coronary syndromes based on risk stratification by biochemical markers: an idea whose time has come. Circulation 1998; 98 (18):1831-3.
Download article: http://circ.ahajournals.org/cgi/content/full/98/18/1831

P/S: The image uploaded is a picture of creatine kinase from a public domain image in wikipedia