Pleural fluid: Thoracentesis


  • Serum glucose and serum LDH and total protein
  • Pleural fluid labs:
    • Tube #1 (Chemistry): glucose, total protein, LDH
    • Tube #2 (Microbiology): culture and susceptibility (includes Gram stain), Fungal stain, AFB
    • Tube #3 (Hematology): Cell count and differential
    • Other: Cytology, pH

Light’s Criteria. Used to distinguish transudative from exudative pleural effusions [thoracentesis to Dx]

Exudative pleural effusions meet at least one of the following criteria, whereas transudative pleural effusions meet none:

  1. pleural fluid protein/serum protein > 0.5
    • protein = albumin
  2. pleural fluid LDH/serum LDH > 0.6
  3. pleural fluid LDH more than two-thirds normal upper limit for serum LDH
    • pleural fluid LDH > (2/3) X (upper limit of normal serum LDH; N: 100-190)
      • (2/3) X 190 = 126.6666 so 127.
        • Therefore: pleural fluid LDH > 127

If a patient has an exudative pleural effusion, the following tests on the pleural fluid should be obtained:

  • description of the fluid
  • glucose level
  • differential cell count
  • microbiologic studies
  • cytology: help detect malignancy source – look for malignant cells

Underlying causes of Exudative process: increase in capillary permeability, which allows for protein and LDH to pass into the pleural fluid.

Example of Exudative causes: (typically inflammatory processes)

  • infections (TB, pneumonia) – may develop an empyema [infected pleural effusion: need chest tube to drain b/c antibiotics cannot penetrate and clear on their own]
  • PE
  • Dressler’s syndrome (post-CABG)
  • Autoimmune (Rheumatoid Arthritis, SLE, scleroderma)
  • Malignancy (neoplasms: lymphoma, lung cancer, breast cancer)
  • Other: Sarcoidosis, esophageal rupture (high amount of amylase in the pleural fluid), pancreatitis

Transudate fluid:

Underlying causes of Transudate process: (change in Starling forces) increase in hydrostatic pressure and/or decrease in oncotic pressure.

Example of causes: CHF, Nephrotic syndrome, liver failure, Cirrhosis



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