Introduction
- Lactate Dehydrogenase (LDH) is an intracellular enzyme present in almost all body tissues.
- It plays an important role in carbohydrate metabolism by catalysing the reversible conversion of lactate to pyruvate.
- LDH is highly concentrated in the heart, liver, kidneys, skeletal muscles, lungs, brain, and red blood cells.
- Under normal conditions, only a small amount of LDH is found in blood serum.
- When tissue damage or cell destruction occurs, LDH is released into the bloodstream, causing serum levels to rise.
- The LDH test is used to detect and monitor tissue injury and various pathological conditions.
- It is considered a non-specific biochemical marker because elevated LDH may occur in many diseases.
- LDH estimation is commonly used in diagnosing liver disease, myocardial infarction, hemolytic anaemia, muscle injury, and malignancy.
- The enzyme exists in five isoenzyme forms, each associated with different tissues.
- Interpretation of LDH values is usually done along with other laboratory investigations for accurate diagnosis.
Principle
- The LDH test is based on measurement of enzyme activity through spectrophotometry.
- In laboratory practice, LDH catalyzes conversion of lactate into pyruvate while reducing NAD⁺ to NADH.
- The increase in NADH concentration is measured at 340 nm wavelength, because NADH absorbs ultraviolet light strongly.
Principle Reaction:
Lactate + NAD⁺ → Pyruvate + NADH + H⁺
The rate of increase in absorbance is directly proportional to LDH activity in serum.
Specimen
The preferred specimen for LDH estimation is:
- Serum
Important precautions:
- Avoid hemolysis because red blood cells contain high LDH concentration.
- Hemolyzed samples may falsely increase results.
- Serum should be separated quickly after collection.
Sample storage:
- Fresh serum gives best results.
- Refrigeration may be used for short storage.
Reagents
| Reagent | Composition | Concentration | Function |
|---|---|---|---|
| R1 | Tris Buffer (pH 7.5) | 100 mmol/L | Maintains optimum pH for enzyme reaction |
| R1 | Pyruvate | 2.0 mmol/L | Acts as substrate for LDH reaction |
| R2 | NADH | 1.66 mmol/L | Participates in oxidation-reduction reaction |
Procedure
| Step | Reagent Blank | Calibrator | Sample |
|---|---|---|---|
| Working Reagent | 1.000 ml | 1.000 ml | 1.000 ml |
| Distilled Water | 0.020 ml | — | — |
| Calibrator | — | 0.020 ml | — |
| Sample | — | — | 0.020 ml |
- Mix gently and incubate for 1 minute at 37°C.
- Place cuvette in analyzer and record initial absorbance against reagent blank.
- Measure absorbance exactly after 1 minute, 2 minutes, and 3 minutes.
- Calculate ΔA/min (change in absorbance per minute).
Calculation
Formula Using Factor Method
LDH (U/L) = 8095 ×ΔA/min
- ΔA/min = Change in absorbance per minute
- 8095 = Kinetic factor at 340 nm wavelength
Normal Reference Range of LDH Test – 225 – 450 U/L
Clinical Significance
1. Myocardial Infarction (Heart Attack)
LDH rises significantly after myocardial infarction because damaged cardiac muscle cells release LDH into circulation.
- LDH begins to rise 24–48 hours after infarction
- Peaks within 2–3 days
- Remains elevated for 5–10 days
A characteristic finding is LDH-1 becoming higher than LDH-2, called the LDH flip pattern, which strongly suggests cardiac muscle injury.
2. Liver Diseases
LDH is elevated in many liver disorders because hepatocytes contain large amounts of the enzyme. Increased levels are seen in:
- Acute hepatitis
- Chronic hepatitis
- Liver cirrhosis
- Hepatic necrosis
- Obstructive jaundice
Marked elevation often indicates active liver cell destruction.
3. Hemolytic Anemia
Red blood cells contain abundant LDH, especially LDH-1 and LDH-2.
In hemolytic anemia, destruction of erythrocytes releases large amounts of LDH into serum.
High LDH is therefore an important marker for:
- Hemolytic anemia
- Mismatched blood transfusion reaction
- Severe megaloblastic anemia
4. Skeletal Muscle Diseases
Skeletal muscle injury causes increased LDH, particularly LDH-5. Elevated levels occur in:
- Muscular dystrophy
- Muscle trauma
- Vigorous exercise
- Myositis
- Crush injury
The level often correlates with severity of muscle damage.
5. Kidney Diseases
LDH may rise in renal disorders such as:
- Renal infarction
- Acute kidney injury
- Severe renal ischemia
Renal tissue damage causes leakage of intracellular LDH into blood.
6. Pulmonary Disorders
In lung diseases, LDH elevation may occur due to tissue injury. Common causes include:
- Pulmonary embolism
- Pneumonia
- Lung infarction
- Severe respiratory distress
LDH-3 is often increased in pulmonary involvement.
7. Malignancy and Cancer
LDH is widely used as a marker in oncology because rapidly growing tumors release LDH due to increased cell turnover.
Raised LDH is seen in:
- Leukemia
- Lymphoma
- Liver cancer
- Germ cell tumors
- Metastatic cancers
High LDH often indicates:
- Tumor burden
- Aggressive disease
- Poor prognosis
- Response to chemotherapy
8. Severe Infections and Sepsis
During severe infections, tissue destruction and inflammatory damage increase LDH levels.
Elevated LDH may be seen in:
- Sepsis
- Severe bacterial infections
- Viral infections
- Tissue necrosis
LDH is often used as an indicator of disease severity.
9. Tissue Necrosis and General Cell Injury
Any condition causing cell destruction may increase LDH, such as:
- Burns
- Trauma
- Shock
- Ischemia
Thus LDH serves as a general indicator of tissue necrosis.
10. Monitoring Disease Progression
LDH is useful for follow-up in many diseases. Serial measurements help assess:
- Progression of tissue damage
- Recovery after treatment
- Response to chemotherapy
- Severity of organ injury
