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.”
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