Body Fluid AnalysisPulmonology

Pleural Fluid LDH

Normal Range
< 2/3 the upper limit of normal for serum LDH
Sample Type
Pleural Fluid
Fasting
No
Unit
U/L

Function

Lactate Dehydrogenase (LDH) is an enzyme involved in energy production found in almost all body tissues. When fluid accumulates in the space between the lungs and the chest wall (pleural effusion), measuring the LDH level in that fluid is essential for categorization. Pleural fluid LDH is a marker of the degree of inflammation within the pleural space.

Why it is Ordered

The primary reason for ordering Pleural Fluid LDH is to apply Light’s Criteria. This set of rules helps clinicians distinguish between an 'Exudate' (fluid caused by active disease, like infection or cancer) and a 'Transudate' (fluid caused by pressure imbalances, like heart failure). This distinction is the most important step in determining the cause of a pleural effusion.

Associated Conditions

  • Exudative Effusions (High LDH):
    • Pneumonia (parapneumonic effusion).
    • Malignancy (lung cancer, breast cancer, lymphoma).
    • Pulmonary Embolism.
    • Tuberculosis.
  • Transudative Effusions (Low LDH):
    • Congestive Heart Failure.
    • Cirrhosis (Hepatic Hydrothorax).
    • Nephrotic Syndrome.

If the LDH is extremely high (e.g., >1000 U/L), it often suggests empyema (pus in the pleural space), malignancy, or rheumatoid pleurisy.

Check Your Result

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Interactive
U/L

Enter your lab result to see where you stand compared to the standard reference range.

Why Context Matters

A 'bloody tap' (traumatic tap) during the thoracentesis procedure can release LDH from ruptured red blood cells into the fluid sample, causing a false elevation that may lead to a misdiagnosis of an exudate. Additionally, if the patient has received diuretics prior to the fluid collection, the concentration of solutes in the fluid can change, occasionally making a transudate look like an exudate (pseudoexudate). It is always necessary to compare the pleural LDH to the patient's concurrent serum LDH.

Lab ranges are statistical averages, not biological laws. "Normal" for a 20-year-old male isn't normal for a 60-year-old female.

Clinical References

Source-of-truth databases and clinical guidelines for Pleural Fluid LDH:

Research & Guidelines

Related Indicators

DR

Medically Reviewed by Dr. Binoy Babu, MBBS

Board Certified Doctor • 10+ Years Clinical Experience

Dr. Babu is a practicing physician dedicated to empowering patients with clear, actionable medical information. He founded 2opi to bridge the gap between complex lab reports and patient understanding, ensuring everyone has access to a reliable second opinion.

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