A patient is found to have marked elevation of serum creatine kinase (CK) with CK-MB fraction 4% (reference <6%) but total CK of 12,000 U/L. Troponin I is normal. This CK pattern is most consistent with:

• A Isolated skeletal muscle injury (rhabdomyolysis) with proportionate CK-MB elevation from skeletal muscle type 2B fibres ✓
• B Acute myocardial infarction with simultaneous rhabdomyolysis
• C Myocarditis with preferential release of CK-MB isoform
• D CK-MB macro complex artefact from immunoglobulin binding

Explanation

CK-MB as a percentage of total CK provides isoform interpretation. In acute MI, CK-MB% is typically >6-10% of total CK, reflecting disproportionate cardiac muscle CK-MB release. In massive skeletal muscle injury (rhabdomyolysis), total CK is markedly elevated, but most isoform is CK-MM from skeletal muscle. However, skeletal muscle type 2B fast-twitch fibres do contain a small amount of CK-MB, so with very high total CK, CK-MB absolute value may appear elevated but CK-MB% remains <6%. CK-MB 4% with total CK 12,000 U/L and normal troponin-I is characteristic of pure skeletal muscle injury—troponin-I elevation would be expected if myocardial injury contributed significantly. Normal troponin is the key discriminator.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.