Serum LDH isoenzyme analysis in a patient with chest pain shows a flipped LDH-1/LDH-2 ratio (LDH-1 > LDH-2) at 48 hours. LDH-1 is rich in H (heart) subunits. In contrast, elevated LDH-5 is most specific for which condition?
- A Pulmonary infarction — LDH-5 is the predominant lung isoenzyme
- B Hemolytic anemia — RBCs release LDH-5 predominantly
- C Hepatocellular damage or skeletal muscle injury — LDH-5 is the MM-rich isoenzyme predominant in liver and skeletal muscle ✓
- D Renal tubular necrosis — proximal tubular cells are LDH-5 rich
Correct answer: C. Hepatocellular damage or skeletal muscle injury — LDH-5 is the MM-rich isoenzyme predominant in liver and skeletal muscle
Explanation
LDH exists as five isoenzymes composed of different proportions of H (heart) and M (muscle) subunits (tetramers). LDH-1 (H4) is highest in heart, RBCs, and kidney; the 'flipped' LDH-1>LDH-2 ratio is characteristic of myocardial infarction (used before troponin era). LDH-5 (M4) is predominantly found in hepatocytes and skeletal muscle. Elevated LDH-5 indicates hepatocellular damage (hepatitis, hepatic congestion) or rhabdomyolysis/myopathy. Pulmonary infarction elevates LDH-3 (and sometimes LDH-1). This isoenzyme pattern helps differentiate the tissue source of LDH elevation.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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