Final answer:
The rise in CPKmb fraction does not correlate better with infarct size than troponin levels; it is actually the other way around. Troponins are more cardiac-specific and remain elevated longer, making them preferable for assessing myocardial infarction.
Step-by-step explanation:
The statement that the rise in CPKmb fraction correlates better with infarct size than does troponin level is False. Troponin levels are now considered the preferred biomarker for indicating myocardial infarction (heart attack) because troponins, including troponin T and I, are more specific to the heart muscle and remain elevated for a longer period. This allows for a larger diagnostic window.
CPK, or creatine phosphokinase, exists in three isoenzyme forms, with the CPK-MB isoenzyme being one of them. While CPK-MB is indicative of heart attack, its levels in plasma remain higher for only 1-2 days following an infarct. Troponin, on the other hand, increases shortly after a heart attack and remains elevated, providing a more sustained indicator of myocardial injury. It is significant to mention that after a myocardial infarction, the concentration of CPK after the first attack is 10 times more than normal and can increase significantly if subsequent attacks occur in a short period. However, this rapid rise and fall of CPK levels makes it less useful than troponin for indicating the size and timing of a heart attack.