Lactate dehydrogenase (LDH) has five isoenzymes (LDH1–LDH5). In acute myocardial infarction with a 'flipped LDH' pattern, which isoenzyme ratio becomes abnormal?

• A LDH5 > LDH1
• B LDH3 > LDH1
• C LDH2 > LDH4
• D LDH1 > LDH2 ✓

Explanation

Normally LDH2 > LDH1 in serum. In acute myocardial infarction, the cardiac isoform LDH1 (H4 tetramer, composed of four H subunits — enriched in heart and RBCs) rises markedly, causing LDH1 to exceed LDH2, producing the characteristic 'flipped LDH' pattern. This pattern persists 48 hours to 2 weeks after MI, providing a late diagnostic window historically used before troponin assays. LDH5 (M4 tetramer) is predominantly hepatic/skeletal-muscle derived.

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

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