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Pleural Effusions

Author: Eddie Qian


There is a normal influx of fluid into the pleural space due to leaky capillary membranes and the pleural space’s negative pressure. This fluid is constantly reabsorbed by lymphatics. An imbalance in the system will result in accumulation.

Some examples:

  • Increased Influx: increased filtration from the capillaries from high intravascular hydrostatic pressure (i.e., heart failure, renal failure, volume overload) or low intravascular oncotic pressure
  • Other liquid entry into the pleural space through anatomic deficits: CSF, chyle, urine, blood, ascites (the diaphragm is naturally porous and pleural effusions with ascites are not uncommon)
  • Decreased Efflux: obstruction of the parietal pleural stoma (from protein or cellular debris in exudative pleural effusions)
  • Increased systemic venous pressure (lymphatic system drains into systemic venous circulation, so if there is high venous pressure this prevents the lymphatics from draining appropriately)



  • CXR: lateral decubitus position (if effusion moves with gravity, suggests free flowing)
  • Ultrasound: assess size, location, loculations (fibrinous septations which may prevent simple drainage)
  • CT Chest with contrast (not always indicated; helpful to eval septations)

Thoracentesis (see Procedures section)

Will it change management?

If septated or empyema,  consider pulm consult for chest tube.


  • Always: Pleural LDH, protein, cell count/diff, culture
  • Don’t forget serum LDH & protein!
  • Consider: pleural cytology, hematocrit, triglycerides, glucose, amylase


Lights Criteria >=1 of the following = exudative
pleural protein : serum protein > 0.50
pleural LDH : serum LDH > 0.60
pleural LDH : upper limit of normal of serum LDH > 0.66
  • Lights criteria: >=1 of the following to be considered an exudative effusion
  • Pleural to serum protein ratio > 0.5
  • Pleural to serum LDH ratio > 0.6
  • Pleural LDH > ⅔ upper limit of normal range of serum LDH
  • Transudative: CHF exacerbation, hepatic hydrothorax, atelectasis (caused by increased intrapleural negative pressure), hypoalbuminemia, renal failure
  • Exudative: infections (bacterial, TB, fungal), malignant, rheumatologic, PE

Pearls about initial tests:

  • Protein: >5 think TB or malignancy, =< 0.5 think urine, CSF, peritoneal dialysate
  • Glucose: =< 60 think about malignancy, TB, rheumatologic, and less specific hemothorax or parapneumonic
  • Cell count/differential: polys represent an acute process, monocytes represent a chronic process, lymphocytes think about TB or malignancy, eosinophils think about air/blood, TB, malignancy, asbestos, drugs
  • Two separate processes may co-occur and a transudate may mask an exudative effusion; if  concerned for this and you have clinical stability, trial diuresis prior to thoracentesis.

Lights Criteria Original Paper

Last update: 2022-02-12 21:53:17