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Lactate dehydrogenase (LDH) is a cytoplasmic enzyme that catalyzes the conversion of pyruvate ↔ lactate in anaerobic glycolysis.

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It is ubiquitous, present in virtually all tissues, and exists as five isoenzymes (LDH-1 to LDH-5), reflecting tissue origin.

Physiological Role

• Key enzyme in energy metabolism (glycolytic pathway).
• Isoenzyme distribution:
  • LDH-1, LDH-2: Heart, RBCs.
  • LDH-3: Lungs.
  • LDH-4: Kidneys, pancreas, placenta.
  • LDH-5: Liver, skeletal muscle.

Serum LDH levels are normally < 250 U/L (lab-specific variations).

Pathological Elevation (Etiopathology)

Because LDH is present in many tissues, elevated serum LDH is a nonspecific marker of cellular injury or high cell turnover:

1. Hematologic causes
   • Hemolysis (autoimmune, megaloblastic anemia).
   • Hematologic malignancies (leukemia, lymphoma).
2. Oncological associations
   • Germ cell tumors: Prognostic marker, included with AFP & β-hCG in IGCCCG classification.
   • Lymphoma: Elevated LDH correlates with tumor burden and aggressiveness; incorporated in International Prognostic Index (IPI).
   • Other malignancies: Non-specific but may rise in advanced/metastatic disease.
3. Other systemic causes
   • Myocardial infarction (historically used before troponins).
   • Liver disease (hepatitis, cirrhosis, metastatic infiltration).
   • Muscle injury (trauma, rhabdomyolysis).
   • Pulmonary embolism, infections.

Radiological Perspective (Grainger & Allison’s)

• Oncologic imaging: LDH is not diagnostic radiologically, but persistent or rising LDH in lymphoma or germ cell tumors should trigger imaging (CT/MRI/PET-CT) to evaluate for recurrence or progression.
• PET-CT: Particularly in lymphoma, elevated LDH often correlates with high metabolic activity on FDG PET.
• Testicular tumors: CT abdomen/pelvis and chest when LDH is elevated, even if AFP/β-hCG are normal, because it reflects tumor burden.

Clinical Applications