Serum troponin I (cTnI) is measured in a patient presenting with chest pain. Which property makes cTnI a superior biomarker of myocardial injury compared to total CK?

• A cTnI has higher catalytic activity than CK isoenzymes
• B cTnI rises faster than all other cardiac markers after injury
• C cTnI does not cross the blood-brain barrier and accumulates selectively in serum
• D cTnI is cardiac-specific and is not expressed in detectable quantities in healthy skeletal muscle ✓

Explanation

Cardiac troponin I (cTnI) is encoded by a gene that is exclusively expressed in cardiac muscle; the isoform produced is immunologically distinct from slow and fast skeletal muscle troponin I isoforms. In contrast, total CK is expressed in brain, smooth muscle, and skeletal muscle. This cardiac specificity means any detectable rise in cTnI above the 99th percentile reference limit is virtually diagnostic of myocardial cell necrosis, making it the preferred biomarker for acute myocardial infarction diagnosis.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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