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Mariam Swidan: Why Is LDH the ‘Tattletale’ of the Lab?
Mariam Swidan/LinkedIn
May 29, 2026, 02:35

Mariam Swidan: Why Is LDH the ‘Tattletale’ of the Lab?

Mariam Swidan, Biomedical Science Graduate, Clinical Laboratory Professional, shared a post on LinkedIn:

“Clinical Lab Concepts 8: LDH

Why is LDH the ‘tattletale’ of the lab?

It’s non-specific, but when it rises, it tells a story of cell destruction you can’t ignore.

Lactate Dehydrogenase (LDH) is one of the most widely used – yet nonspecific – biochemical markers in laboratory medicine.

LDH is a cytoplasmic enzyme that catalyzes the interconversion of pyruvate and lactate (pyruvate is in a reversible reaction with lactate) – an essential step in anaerobic glycolysis

LDH Isoenzymes

LDH exists as 5 isoenzymes (LDH-1 to LDH-5) composed of:

  • H (heart) subunits
  • M (muscle) subunits

Each isoenzyme has a characteristic tissue distribution:

  • LDH-1 (H4): heart, RBCs
  • LDH-2 (H3M1): predominant in serum, also in RBCs and the reticuloendothelial system
  • LDH-3 (H2M2): lungs
  • LDH-4 (H1M3): kidneys, pancreas, placenta
  • LDH-5 (M4): liver, skeletal muscle

In normal serum: LDH-2 > LDH-1

In hemolysis: LDH-1 may exceed LDH-2 (‘LDH flip’)

Why is LDH elevated in hemolysis?

Red blood cells rely entirely on anaerobic metabolism, making them exceptionally rich in LDH.

When RBCs rupture:

  • Intracellular contents are released into the plasma
  • LDH levels rise in serum

This makes LDH a highly sensitive marker of cell destruction

LDH in Hemolytic Anemia

Intravascular hemolysis (IVHA):

RBC destruction occurs directly within the circulation

Marked LDH elevation

Due to direct intravascular release into plasma

Often associated with:

  • Hemoglobinemia
  • Hemoglobinuria

Extravascular hemolysis (EVHA):

RBCs are destroyed in the spleen and liver (macrophages)

Milder LDH elevation

Because breakdown is contained within the RES

The higher the LDH, the more likely an intravascular component is present.

Correlation with Other Hemolysis Markers:

LDH becomes most powerful when interpreted alongside:

  • Low haptoglobin because it binds free hemoglobin
  • Elevated indirect bilirubin due to heme breakdown
  • Increased reticulocyte count reflecting bone marrow response

Together, this forms a classic hemolytic profile

Limitations

Despite its high sensitivity, LDH lacks specificity

It may also be elevated in:

  • Liver disease
  • Myocardial injury
  • Muscle damage
  • Malignancy

Always interpret LDH within a clinical and laboratory context

Takeaway

  • LDH is not diagnostic on its own – But in the right context, it becomes a powerful indicator of hemolysis.”

Mariam Swidan: Why Is LDH the 'Tattletale' of the Lab?

Find more posts featuring Mariam Swidan on Hemostasis Today.

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