A 58-year-old man presents 6 hours after acute chest pain. Troponin I is elevated. Which isoenzyme pattern of CK would be most diagnostically specific for myocardial injury at this time point?

• A CK-MB fraction >6% of total CK activity ✓
• B CK-MM predominating with no CK-MB rise
• C CK-BB elevation alone
• D Total CK elevation with CK-MB <5% of total

Explanation

CK-MB constitutes >6% of total CK activity is the diagnostic cut-off for myocardial origin. CK-MB rises at 4–6 hours, peaks at 18–24 hours and is cardiospecific when the fraction exceeds 6%. CK-MM is the skeletal muscle isoform and its predominance without CK-MB elevation does not indicate myocardial injury. CK-BB is the brain isoform and rarely reaches circulation in significant amounts. Elevation of total CK with CK-MB below 5% suggests skeletal muscle disease.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.