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lights criteria + thoracentesis


Usually, the visceral and parietal pleura have a thin later of fluid that helps the two layers slide over each other during respiration.

A pleural effusion is excess fluid that accumulates within the pleural space. This fluid can be anything - blood, pus, urine, chyle - but is usually transudate or exudate.

  • Transudate: due to inbalance between hydrostatic pressure (pressure in capillary beds pushing fluid and solute out) and oncotic pressure (pressure pulling fluid into capillaries/preventing fluid from leaving)

    • Examples: Heart failure, hepatic hydrothorax, atelectasis

  • Exudate: due to inflammation of the pleura

    • Examples: Malignancy, ARDS, lung abscess, sarcoidosis

Other less common types of fluid:

  • Pus: from empyema from infections

  • Blood: trauma

  • Chyle: rupture of thoracic duct

  • Urine: 2/2 hydrothorax

It can be seen on upright CXR, where you will have blunting of the costophrenic angles/loss of silhouette. It’s usually dense but can be cloudy appearing (esp in patients on vent- the effusion layers posteriorly)

Steps to evaluating effusions

  1. Get CXR. If you see an effusion that is large, order chest ultrasound to evaluate further

  2. Chest ultrasound will tell you whether effusion is “tappable” (e.g. can you do thora?)

  3. Once determined it’s amiable to thoracentesis, you can:

    1. Do thoracentesis yourself

    2. Order IR or US thoracentesis

      1. Hold chemical anticoagulation just like you would for any procedure

  4. Once you have pleural fluid, the following lab studies can help you depending on what you’re looking for:

    1. To determine transudative vs exudative: serum LDH, serum total protein, pleural LDH, pleural protein in order to calculate lights criteria (see below)

    2. If you see cloudiness: check pleural triglycerides (often chylothorax/chyle leak)

    3. If you see blood: pleural Hgb/Hct

Light’s criteria


Perform a thoracentesis


  • Unilateral pleural effusions are mostly exudative, while bilateral effusions are often transudative (e.g. CHF)

  • CHF effusions are often greater on R than on L

  • A MASSIVE unilateral effusion = think malignancy

  • A loculated effusion = think empyema until proven otherwise

  • In effusions caused by ATX, there is mediastinal shift towards effusion