A 55-year-old man is admitted with chest pain. Serial troponin I measurements at 0, 3, and 6 hours are 0.02, 4.8, and 9.2 ng/mL respectively. Which property of cardiac troponin I makes it superior to CK-MB for late presentation (48–72 hours after onset)?

• A Troponin I appears in blood earlier than CK-MB after myocardial injury
• B Troponin I is unaffected by skeletal muscle disease, unlike CK-MB
• C Troponin I remains elevated for 7–14 days because of release from the structural myofibrillar pool ✓
• D Troponin I has a higher molecular weight allowing slower renal clearance

Explanation

Cardiac troponin I has a biphasic release: an early cytosolic pool peak within 6–12 hours, followed by a prolonged elevation for 7–14 days from degradation of the structural myofibrillar-bound pool. This sustained elevation makes it far superior to CK-MB (which returns to baseline within 48–72 hours) for diagnosing late-presenting myocardial infarction. CK-MB can be elevated in skeletal muscle disease, but this is a secondary advantage, not the reason for prolonged detectability. Molecular weight and renal clearance do not govern the 14-day window.

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

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